I found a bug on my bedroom floor. Is this a bedbug?
To answer your question this is very likely NOT a bedbug.


See this answer for a positive ID of (and more info for) a bedbug.


However, based on the size and body shape of your specimen, it appears you've found another blood sucking paristic pest: a flea.





 Sources: Here, here and here. 

 General Info: 


Fleas are small flightless parasitic insects (order Siphonaptera) that consume the blood of mammals (including humans) and birds. 
Over 2,500 species of fleas have been described worldwide.
Adults are 1.5-3.3mm long, narrower than wide and brown (or reddish brown). [1,2]
Flea larvae are worm-like (vermiform) in shape with a sparse covering of bristles.  They rarely live on the body of their host.  Instead, they are usually found in its nest or bedding where they feed as scavengers on organic debris (including adult feces).




 Likely Species: 


I'm assuming you have a pet (cat or dog perhaps). If that is the case, you're likely looking at a cat flea or dog flea (depending on your locaiton). From UF IFAS:


  The most common [American] flea is the cat flea, Ctenocephalides felis  (Bouché). The adult cat flea, unlike many other fleas, remains on the host. Adults require a fresh blood meal in order to reproduce.
  
  The dog flea, Ctenocephalides canis (Curtis), appears similar to the cat flea, but is rarely found in the United States. Cat fleas are commonly found on both cats and dogs in North America, while dog fleas are found in Europe. 
  
  The two species are distinguished by a slight morphological difference which is detectable only under high magnification.




 Cat flea (Ctenocephalides felis) from University of florida IFAS 

Note: both species will parasitize multiple species (including both cats and dogs as well as humans), and their distinction (as mentioned above) is more dictated by geographic location. However, both can be found worldwide. 




Linardi &amp; Santos (2012)1 have an excellent publication for correctly identifying between the dog and cat flea. 



For More General Information See:


  Wikipedia
 - Terminix
 - Orkin
 - Univ. of Evansville
 - Purdue University
 - UFL IFAS (cat fleas or dog fleas)
 



 Fleas vs. Bed Bugs 

See this Terminix page for "What is the difference between bed bugs and fleas?"





Your specimen is almost certainly a flea because:


The body of the insect in your picture is thin but vertically "tall" (or "laterally flattened"). In other words, it appears to be narrow like a flea.


A bed bug, on the other hand, is wide and vertically very flat.


Though, an engorged adult can appear to be more narrowly long when engorged on blood. See here.

Fleas are narrow so that they can easily maneuver through a host's fur/hair/feathers; while a bedbug is flat to be able to effectively hide in crevices. 

The size (3-4 mm) is more appropriate for a flea.


Although some larval stages of bed bugs can range between 2.5 - 4 mm, these larval stages still retain the relatively flat appearance of the adult (except when engorged).

Notice the reflection toward the posterior end of the specimen in your photo. This is not present on bed bugs. This light is reflecting off the flea's pygidium (or sensilium). 

 
Some fleas have bristles on the posterior end of their body. Bed bugs do not. The insect in your picture has these posterior "hairs."
Fleas have powerful hind legs for jumping that stay relatively close to the body, while bed bugs have very skinny legs that spread laterally from the body. 


When bedbugs become engorged and appear more like the insect in the question picture (i.e., not flat), their abdomen grows posteriorly but their legs do not extend very far back. 
Notice when I brighten the OP's picture from the question you can see the light reflecting on the wide posterior femur that is both close to the body and extends relatively posteriorly as compared to the overall abdomen:



Location &amp; Behavior: Although both fleas and bedbugs are generally nocturnal, both can be found during the day. However, they would generally be found in different places:


Bed bugs are likely to be digesting meals on the piping of your mattress, in and around the bed frame or box spring, along the baseboards and creeping around in other well-hidden cracks and crevices
Fleas, however, (like your specimen) are more likely to be found making themselves at home in animal sleeping areas or lurking around in your upholstery or carpeting, waiting to hop on for a bite. [Source]





Should You Still be Concerned?


Some fleas are well known to transmit diseases (e.g., plague, murine typhus, etc., and a host of issues for your pets) unlike bedbugs. So really, fleas could potentially be more harmful. 


Unlike bedbugs, however, vacuuming goes a long way for managing fleas on bedding, carpets, and furniture. 
However, one flea can lay between 150 and 300 eggs every week, so do not wait!





 Works Cited: 

1  Linardi, P.M. and Santos, J.L.C., 2012. Ctenocephalides felis felis vs. Ctenocephalides canis (Siphonaptera: Pulicidae): some issues in correctly identify these species. Revista brasileira de parasitologia veterinária, 21(4), pp.345-354.