shorter periods for 2 months after losing virginity

im 18 and i have a 27 day cycle. I had periods on 23rd march, 19th april and 16th may. I had sex for the first time on 15th april, that was on the 23rd day of the cycle, so im assuming it was safe. And second time on 4th may which was on the 13th day of the cycle, which would be when im ovulating or begin to ovulate. Ive used condoms both times. And it didnt seem like they tore (but who knows) In april and may i had 3 day long periods whereas previously i used to have it for 5-6 days. And ive been having cramps since 14th may ish and even today. Even though im on the 4th day of my cycle and my period is over, and i had heavy bleeding. Since the implantation/spotting is also supposed to be 2 weeks after ovulation/fertilization (which also coincides with the date when my period's due 4th may-16th may=2ish weeks), im slightly worried whether this could be implatation bleeding and not periods. However it was heavy bleeding whereas inplantation bledding is pinkish/brownish discharge. Can someone assure me its okay to have shorter periods after having lost one's virginity. Or should i be worried (am i pregnant??)? Also, one week before (day 22-27) and after(day 1-6) periods is the safe period, right? Day 7-21 consists of fertile period, right? How safe is it to have unprotected sex (without condoms or birth control or contraceptives) during the safe time? Please help. Im much worried! Thanks.