sore abdomen

Okay so my SO and I have been having sex everynight for the past week. he came in me on tuesday and last night as well. Tuesday I had globs of stretchy cm seemed very fertile and Friday I had watery cm with a positive ovulation test. I don't understand why. my other issue is my lower middle abdomen is so sore and tender. could this be a sign? Or maybe just a worm out feeling from sex ?