I have got a very painful small lump under left arm pit, and am very scared it is something serious. please help! it came up yesterday.

Not cancer: When cancer spreads to a lymph node the node SLOWLY grows larger (it won't suddenly appear) and is not tender or painful. Your lump is a small boil (infected hair follicle). This is a skin abscess(collection of pus) caused by Staph aureus usually. Warm soaks may bring it to a head so it drains. If it continues to enlarge see a physician because MRSA is a possibility, which can get worse.