Hi there and welcome to MedHelp's forums. Arm pit lumps can happen for a variety of reasons. Yours sounds sudden, large and infected. Here is an overview of armpit lumps https://www.healthline.com/health/armpit-lumphttps://www.healthline.com/health/armpit-lump
Lumps can be viral, bacterial or have a more serious cause. I would call your doctor on the phone for advice and most likely, they will work you into their schedule to be seen at the earliest time they can.
If you do not have a regular doctor, urgent care would be sufficient and a cheaper option that the emergency room. Often, this is not serious but requires attention to treat it. Let us know the outcome!