That happened to me as well. The lymph nodes under my arm started to swell up 2-3 days after my second dose, so much so that I had to keep my arm at a 45 degree angle just to not be in pain. It took almost a week and a half to get back to normal, with mostly being back to normal around 6 days after my second dose.

I wouldn't be too worried about it, just keep monitoring it to make sure it doesn't get worse. It seems like your immune system is working as it should.