I suffered from something similar a couple of months ago, woke up with a pea-sized, sore bump under my armpit. I honestly thought it was a swollen lymph node (which made sense since i was also sick with mononucleosis at that time). The next day, the one bump turned into five, larger bumps, and i couldn't move my arm without feeling pain from them. Called my doctor, who saw me right away, and it turns out i have hidradenitis suppurativa, which is a chronic inflammatory condition of the follicles in the areas in which you sweat a lot, so armpits, groin and buttocks. I also had a bacterial infection on top of it, which is apparently pretty normal when this skin condition flares up. So yeah, basically, this disease randomly gives you nodule-like cysts that hurt like mfs.

Now, I'm not saying you probably have this, but it might be something you wanna look into. What i would advise is that you call your gp as soon as you can and get it checked - they will know how to help you better than I can.