What Causes Frequent Urination With Discomfort In Womb?

i am 28 yrs old, got 2 children 3.5 yrs and 1.8yrs old. I am having discomfort in the womb area as though a baby is moving in there. I, having frequent urination, and have alight pain and discomfort while walking. Pregnancy test is negetive. GP has prescribed antibiotics and taken a swab for testing. Gp said it could be an infection in the womb. I want to know whats the reason for me to get it. I am married and we dont have regular sex.