What is causing a cat urine smell in my bathroom?
Look for leaks, the most likely culprit is a failed seal on the toilet.  If the toilet rocks or moves that will damage the seal and cause leaks.  Even if it was installed tight to the floor it is possible the seal (usually a wax ring) was not large enough, or was not replaced by a previous homeowner after removing the toilet.  Also check if the toilet is caulked to the floor, this should never be done.  It just traps leaks under the toilet where they will ruin the floor without you noticing.

Remove the toilet and inspect underneath.  If the floor isn't rotten and the flange is not broken replace the wax seal and remount.  Or hire a plumber to do the same. 

Also even if you don't have cats the previous homeowner may have and moisture from showering is bringing out the smell.  Think about where you would put a catbox and inspect and sniff carefully.  It is common for cats to pee against the baseboards and the urine to seep under the flooring.  If water gets on the floor it will bring up the smell.  

In one case I had to tear a room back to the studs to get rid of the odor.