I'm 14 And My Period Ended July 7th 2018 And

I'm 14 and my period ended July 7th 2018 and its noe Aug 25th and I haven't started one yet, I recently went to my boyfriend's and he got sperm inside my vagina, I'm not on birth control. I did spot about 2 weeks ago, but I am still concerned. My periods are known to be really late so I'm not sure if its that or I may be pregnant.I haven't had any pregnancy symptoms besides the usual backpain. Please I need help!