College exams. Life after graduation has its own stresses but nothing was more stressful to me than having 6-7 exams coming up and having to spend so much time and effort preparing for stuff I don't care about that much.
At some point it got so bad I'd wake up a few times at night drenched in sweat a day before exams. Yeesh. I guess it was partly my fault for taking it too seriously but I couldn't switch that off.