What Causes Bleeding After Having Unprotected Sex?

good morning..i am 20 years old,my period comes every 2nd or 3rd day of the month and it lasted like 5-7 days,,i had unprotected sex since april 18-may 2..but the thing is i got light bleeding that lasted 3 days on 3rd week of april so its like april i had 2 menstruation then on may 10 i had light brown spotting..and this month since yesterday i had very lil light brown spotting...i feel back pain,sleepy,tiredness,and sometimes belly cramps since 1st week of may..wat u think the cause of this...??thank you doctors..