What Causes Scanty Menstrual Periods?

Hello, my boyfriend just stuck his penis in an inch and pulled out because i was not ready. I havn't had my period for 3 months and already took pregnancy tests'( the blood and urine) and both came out negative. I'm wondering why i havn't gotten my period but i've also had somedays where some of my period will come out, then stop.... should i be worried