Is there an explanation for concepts or situations that are repeated within a short amount of time? I know that the title is not really clear. That's because I don't know what i'm searching for, unfortunately.
Let me explain with an example:
Yesterday i was watching a youtube video , more precisely a lecture from MIT. Looking through the comments i found out that the professor from that lecture studied computer science at the University of Waterloo. I have never heard about the University of Waterloo before (until yesterday). 
Today, while checking my emails i was surprised to see that i got a suggestion to read an answer to a question : "How valuable is the University of Waterloo's computer science program? "
I know that with computers may not be so mysterious since they know what content we see on the internet and may use it for making suggestions.
The thing is that these kind of situations occur really often even without technological "help".

For example:
Some time ago my sister told me about neutron stars. It was fascinating.
The next day, during a lecture, my physics professor used neutron stars as an example for a problem.

I had never heard about neutron stars before my sister told me about them and i was shocked to hear my professor that day. I haven't heard about those stars since then.

Or even everyday things: there is a song you didn't listen to for a very long time. Let's say a year. One day you think about that song. Some hours later you hear it playing randomly in a tv show you are watching.

My question is : do these situations have a name ? 
It happens so often that i already gave it a name but i would like to know if there is some official version

Do they have an explanation from a psychological point of view or they are just fortunate coincidences?
Short answer
The repeated encounter of a word or object after just having been introduced to it is linked to the frequency illusion, also called the Baader-Meinhof phenomenon.

Background
It may be an example of the frequency illusion:


  The illusion in which a word, a name, or other thing that has recently come to one's attention suddenly seems to appear with improbable frequency shortly afterward


It's also referred to as the Baader-Meinhof phenomenon, colloquially described as


  [T]he phenomenon where one stumbles upon some obscure piece of information—often an unfamiliar word or name—and soon afterwards encounters the same subject again, often repeatedly.


It's also related to things like buying a brand new car that was just newly released and suddenly it pops up all around the neighborhood.  

The Frequency Illusion is thought to be the result of two psychological processes:  selective attention (noticing things that are salient to us, disregarding the rest) (Yantis, 2008), and confirmation bias (looking for evidence that supports a hypothesis and disregarding counterevidence). source: Stanford University  

Reference
- Yantis, Curr Dir Psychol Sci (2008); 17(2): 86–90