What Causes Warm Sensation Around The Abdomen And Back Before Periods?

Hi I am an 18 year old female. For the past few months I have been having a warm (not hot) sensation around my lower abdomen that sometimes travels to my lower back. I have observed that it only happens when I am sitting down. I also noticed that it happens a week or two before my mensturation. I am virgin and am not sexually active. When I went to the doctor blood test, ultrasound both came out normal so I really dont know what to do please help.Its not painful but it has become troublesome