Idk if I’m pregnant or not

Got my period July 21st to the 25th it was regular and on the 28th i had unprotected sex Nd my bf nutted in me , so i waited till my next period day to come and it did but only last 2 days which was August 13th to the 14th the 15th it stop and it looked like old blood but now I’m experiencing light cramping and low back pains