I am having pain in my uterus but I don't know why. What could it be?

Ectopic preg./PID. There are many possibilities, but two of the most important are ectopic pregnancy, and pid or pelvic inflammatory disease. Ectopic pregnancy must be ruled out in cases of uterine/pelvic pain, and is due to a fertilized ovum developing in a fallopian tube. This is an emergency. Pid is also possible, but is more chronic, and can result in infertility. Please see your doctor soon to find out.