Being 4 months pregnant would be rather late to find out. Having sex for the first time can cause your periods to change slightly and can even cause pains. there are many things that can cause uterus pains. If she continues to have uterus pains she needs to see a gynecologist as it could be a variety of different things. If she's not having most of the signs of pregnancy (yes it is possible to feel no other signs but it's rather rare) then she's most likely not pregnant. if she still believes she's pregnant the only way to know is to take a pregnancy test and/or see the doctor. Good Luck.
Next time, use some protection.
