Ive missed 4 periods but my pregnancy test said negative?

ive missed 4 or 5 periods, i lost my virginity around 7 months ago. ive taken multiple pregnancy tests and they have all came up negative. my doctor has me on multiple medications and they tend to change them quite often. i also have had a lot of stress related problems over the last 3 months. could that be the reason why i havent had a period?