I am taking painkiller for my chronic pain, and when i don't take it i got discomfort on my stomach with my backpain. is the discomfort are from paink?

Try stopping: The advil or ibuprofen. If your stomach pain is related to those medicines, your stool maybe dark from the presence of blood due to bleeding from stomach. If your stools start getting lighter in a few days, and your stomach stops hurting than it probably was them. If not, try stopping the muscle relaxant. Separating out the medicine by category/mechanism of action is easy to identify the one.

Irritation ; rebound: Dear bobcat, you did not mention where is the chronic pain you take the meds for, probably back pain, correct? The stomach pain likely represents irritation or injury to the stomach mucosa, which may be potentialy dangerous. Backpain may be a rebound or upregulation of pain sensation with the prolonged use of pain medication. Try any nonpharmacological tools; relaxation , hypnosis , biofeedback.