Penis Hurt during errection.. help?

ok. i'm 15 i'm 8.8 cm errect, and when i masturbate once evrything feels ok.(normal). but when i masturbate for the second time sometimes it hurt. i feel like this stretching feeling. and sometime i get headches. is this normal?the pain doesnt go for like 3- 5 hours... i've told a friend & a cousin and they both told me that maybe it's because my penis is big for my age and that it takes a lot of blood to get an errection. does it make sence to u guys ? or should i see a doctor? (listen i'm telling the truth. this is serious. i'm nervous and i rather ask here first too se if everything is normal or what. please i'm looking for serious answeres.) Thanx for ur Time PPL.