Sore stomach and belly button

I have recently had a chest infection to which I was given Amoxicillin. The day I started taking the antibiotics I was due to come on and I had an extremely light period (no blood, it was just light brown in colour) which for me is highly unusual. After completing the course of antibiotics last weekend, I have since had a really swollen stomach which is extremely uncomfortable and similar to period pains (cramps). As well as this, the inside of my belly button at the top is really sore to touch. I am not sure whether the antibiotics is related to this or not? Or, have I pulled a stomach muscle as I do frequent the gym?