Yes! I've had all kinds of wonky effects from medication, from the working then not that you're describing, to actually making things worse, to making things great one minute then nasty the next. I agree with Rohag, in my experience it took a long time (well over a year for me) to find the right drug/combination of drugs and their dosages to finally get me where I needed to be. At stressful times like exams, I've also been put on additional "booster" medications to help up the effects of my current medication for short periods of time. It's a really long process, so try to be patient, and make sure your doctor knows exactly what's going on with you. In my experience when the meds started working then stopped, it meant we'd struck on the right drug but the wrong dosage, so I had to get my dosage upped several times.

Good luck!