My doc says tiger balm is safe. She even recommended it to use on my forehead for headaches.
She also said a heating pad on the back is okay, just be careful not to burn yourself
I use a regular heating pad on my back most nights to relax. Seems to help and definitely not dangerous!
I had terrible back pain and asked the pharmacist about icy hot patches. He told me NOT to use them because they are medicated, but that I could use deep relief ones because it was just menthol. So I would double check with your doctor before you use it. Especially on your stomach.