What causes a lump on my armpit?

The last week now their have been a little lump on my armpit when i touch it well it feel like a big lump but you really cant see it, It hurn even wen i dont touch it i still can feel it is not that painfull when i dont touch it but it still hurt. I am 20 years old and i remeber a year maybe two years ago i had the same lump on the same under arm but it whet away could I have cancer or HIV what could it be? Please help should I worry..