Help! Lumps in both armpits!?

Right well about 7 months ago both armpits started getting these sore scaly dark red/brown rashes inside them and they would disappear and come back again in a few months and disappear again and this evening I was checking them and I was pressing on my armpits and they were very sore and tender and a few minutes later about 3 quite hard lumps In each armpit appeared. does anybody possibly know what this could be? I do sweat quite abit in the underarms if that could have anything to do with it? But I'm not overweight and quite healthy and I'm 15 years of age. Any help would be great thanks as I am very concerned. Thanks in advance.