I have a lump under left armpit, dr thinks it's only a cyst i go for test tomorrow, it's painful could this be cancer some say cancer doesn't hurt?

Wait for answer: If you are seeing your doctor tomorrow, you should just wait to see what will be found instead of worrying over potentially bad results. Some cancer can indeed hurt -depending on the site involved.

You are right: Cancer commonly is a painless bump/lump. But some tumors can be tender. Armpit is prone to get infection in the hair follicles which can produce a red, tender painful lump but it usually settles down in one to two weeks. Breast cancer, of course can spread to the lymph nodes of the armpit and produce a firm, painless lump there. That is what you need to be sure about.