Mayoona, there is a bunch of weight and pressure on your genital area from the pregnancy. Your belly can cause him to thrust at an angle that punch into that area from inside your vagina. During orgasm your anal sphincter also contracts that will stress muscles that might already be under stress.
Do you experience this when you masturbate as well?
Have you tried different positions? Swap sides. Flip over. Have him stand with you on the bed.