Fell off a horse on tuesday. have had abdominal pain since. today (friday) it is worse. should i have it checked out?

Yes - now.: A fall from a horse is considered "blunt trauma". The most common serious abdominal injury from blunt trauma is a ruptured spleen which might cause exactly these symptoms. Without seeing you, there is no question that the symptoms you are describing merit a real examination and, at the discretion of the examiner, an abdominal ct scan. Please go now. You may be bleeding internally.