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Sometimes it can just be a swollen gland if your body is fighting off an infection. You should see a Dr just to make sure.
This is true. It could also be an ingrown hair. I've gotten a few of those in my armpit, and I could feel a painful pea-sized lump too, but it went away in a few days. I'd guess this is likely what it is, and in that case I'd suggest putting ice on it and try to avoid wearing deoderant (it further clogs the hair duct or w\e). I was scared too, AE, because I know it feels like something serious, but in my case, it really was not at all.
Please don't panic, because you don't know what's wrong yet, and it could be nothing. I'd say just keep an eye on it, and if it's not gone in a few days, then try to go to the doctor (I know you can't right now, but just go when you are able)
One last thing, I don't say this to scare you, but sometimes lumps in your armpit can be a sign of breast cancer... so, if you are a woman, (sorry, I didn't check ur profile for gender) I'd suggest feeling around your breasts and looking out for anything abnormal there - (it's actually possible for men to get breast cancer too, but not likely). Again, not trying to scare you, I'm sure you are fine but its better to be safe than sorry.
In all likelihood, it's nothing. The possibility is there though, so please do whatever you can to get checked out when you are able.