There is a strange bump on my right armpit and it has been there for a week. its also tender to the touch. what could it be?

Lymph node: An enlarged lymph node would be the most common occurrence. Have you been scratched by a cat, had an infection in a finger, or some other inflammation? If this fails to clear up quickly (week or so), or gets worse or is accompanied by other symptoms (fever, fatigue, chills, loss of appetite, etc.), see a doctor and have it examined.