What Causes Missing Periods And Negative Pregnancy Tests?

I am a 23 year old female. I have yet to get my period for the past 4 months and I am starting to get very worried. I took 2 pregnancy tests and they were both negative. I stopped taking my birth control because I missed 4 days in a row, I planned to start taking it when I got my next period but it has yet to come. I am also on antidepressants (zoloft) and pain meds (vicodin) could these medications affect my missed periods?