I have a rash under my arm and on my chest red and irritated slightly bumpy it happened after playing a match of tennis

Rashes: I will assume you have not had a rash like this occur before after playing tennis. Options for the etiology of your rash can include heat rash or friction irritation but also shingles or "bug bites". Please seek medical attention here with a video encounter to show us the rash or with a local provider if the rash persists or worsens.

Several causes: You may have a contact allergy, like poison ivy or reaction to deodorant, or something like shingles. Or, do you have friction in that area for some other reason like obesity or tight clothing? If you don't have other symptoms like fever, severe pain, or rapid spreading, it is reasonable yo wait a fee days for it yo go away. If not, see your doctor or urgent care ASAP.