expelling poop during sex

My boyfriend and I have been together almost 2 years. I was a virgin til we met. I am 39. In spite of my age, I am rather new to the realm of sex. A few (2 or 3) times while having sex I expelled poop without knowing (or feeling it) until we have finished.  It is usually a small amount but enough to be gross and upsetting. This has been rather embarrassing and in all honesty I usually end up crying when my boyfriend rather calmly and gently informs me either not to move til he can clean up, or  he tries to be humorous and says "guess I *&^%$@! the **** out of ya honey."  I have tried to rationalize to both of us that I must really be bearing down when orgasming that I just have no control. Uninformed or inexperienced as I am about sex, is this normal or should I be concerned?