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?