It could be liver related, though you generally get "referred pain" from that...example: gallbladder attacks can hit you in your shoulders...very weird. If you lie down and press below your rib cage, does it feel swollen?
More likely, you have simple gas pains. I've gotten this a lot, especially when I exercise...just comes with the fast.
Ultimately, use your instinct and determine whether it should be looked at by a professional. If the problem persists, definitely see a doctor.
I'm on day 7 and I'm sort of getting what you're getting, only it's mild and accompanied by mild nausea. I always find a good hot bath (maybe with Epsom Salts ) and rest will take care of it.