I Have Recently Started Having Sex, And Every Time I

I have recently started having sex, and every time I have experienced a deep pain, which appears shortly after we start and gradually gets worse. I am small framed whereas my boyfriend is rather large. I dont think it is stretching pain though. Also, afterwards I often have really bad stomach cramping and my insides feel very swollen and tender. Im not sure what is wrong, and if there is anything I can do about it.