Since how long is the pain?
How severe is the pain on the scale of 0-10?
Do you have nausea/vomiting, constipation/diarrhea, bloating, urinary symptoms, increase or decrease with the posture/eating?
Is there any specific point of tenderness?
Any significant past medical or surgical illness?
The pain can occur due to being in same posture for long; so prolonged sitting up in bent position can be responsible for it. Since the pain increases on leaning or bending; if further points it to be of muscle origin. This muscle origin pain theory is further substantiated by the fact that you do not have any visceral symptoms (absent urinary symptoms, nausea, diarrhea, bloating etc).
You can take Motrin and keep a heating pad if the pain gets worse. Avoid those positions which are painful. This is not likely to be serious. This may remit on its own. You may need to see your doctor, if pain intensity increases or any of visceral symptoms start.
Please feel free for your follow up questions.