Hummm, I don't think it's anything serious, I've had a lump under my other arm before, though smaller and that eventually went away, it felt very similar to this one. This lump looks a little sore and it does ache/hurt quite a bit, especially when I touch it or lift up my arm, so I'm thinking something like a cyst personally, it's the most likely option, will keep a close eye on it just in case though, first sign of it doing something crazy and off to the doctors I go~
The hypochondriac in me hates it though 8O
I should probably go and continue some housework now...though my dad said last night he wanted me to do something and he didn't want me doing it like an hour before he comes home and it needed doing early only I can't remember what that was so I hope I did it! :D Something to do with the washing I think.
Got a week off work next week! YAY!