It is a myth (probably perpetrated by men!) that a man's penis can be too big to be accomodated by a woman's vagina, which is designed to accomdate the passage a 6+ pound baby into the world. If you are expeiencing internal pain after intercourse, it is possible that your partner is being a little too 'vigourous', something you may not notice if you are in similar throes of passion. Another reason may be a position that is causing strain on some of your muscles, which again you may not feel at the time. Because you say your pain is in your stomach, I assume you can discount the vaginal discomfort caused by lack of proper relaxation and lubrication. If you try a few more gentle sessions, and the pain persists, it may be worth trying your GP to see if you have any minor internal problems, of which this pain is a symptom.