What could this pain under armpit be?

A few days ago I got a burning/stabbing pain under my right armpit and I don’t know where it has come from. If I jostle around to much it hurts like hell. There isn’t any rash or bruise on the spot but the spot is tender. I know I didn’t tweak a muscle in the gym because I haven’t been in over two weeks and this started almost two days ago. The pain almost takes my breath away. I haven’t done anything that would’ve tweaked that muscle either. I’m so confused and I don’t wanna pay for a doctors appointment if it’s not that serious but I don't know how much longer I can take this pain.
What could this pain under armpit be?
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