Hello Isaackm16 and thanks for posting,

Unfortunately, I am not in a position to be able to say for sure what is causing this lump but glands (lymph nodes) under the arm can sometimes swell up after the COVID vaccine, so this could easily explain what you have noticed. 

I don't think you need to worry, but keep an eye on the lump without poking and prodding it too much (which might make it sore), and see what happens in the next couple of weeks. If it doesn't settle down (go away or start to get smaller) tell your GP so they can make an assessment.  But for now try not to dwell on it and be wary of spending too much time online looking for reasons why lumps crop up as this is likely to keep any worry front of mind and make it worse.

All the best,

Julia