It could be that you have a tilted uterus, most are tilted upwards behind the bladder and pubic bone, some women however have a uterus that is tilted backwards towards the rectum. This is not a bad thing and has no health problems associated with it
(doctors used to think women with a retroverted uterus had difficulty getting pregnant/ this is just not true). During sex, especially if your boyfriend is large, the penis will bang into the uterus and so your sex may be painful and you will be sore afterwards. Tell your boyfriend to be more gentle and also try using different positions for sex, usually the dog-style position is best for this and the missionary position is the worst. This changes the angle of entry and can avoid painful intercourse. If you try these things and the symptoms persist, see your doctor.