What Causes Short Periods?

hi , i am 17yrs old girl.my last to last period was on 28th feb.it ended on 4th Mar.i had sex with my bf on 15th.i did ovulate on 16th. usually my period is of 22days. this time it came on 27th march.initially it was was very light then 6 hours later flow became normal and then again very light on 29th and then stopped same day at 5:30pm followed by severe pain in abdomen.then again on 31st March i did experience normal bleeding from5:30 morning to 5:30 evening.i had back pain,headache and body pain on 2nd April.after that i had leg cramps for 2 days. now i am felling very normal, but i cant understand why my period was only 2 days instead of 3-4 days.also my boyfreind pull out befoe ejaculation.i did two HPT on 27thMarch and 7th April, both came negative.but i still worried. what should i do?i also did take heavy doses of vitamin c on 27th March throughout the dayto avoid pregnancy.please help me doctor.