What Causes Spots In Armpits?

i get spots in my armpits sometimes they are very large, red and pussy they are irritating and unattractive what is it? i also have spots not the same kind on the backs of my legs and bottom. what could b causing this is it just a skin problem or could it be more?? i am overweight and alwasys have been and cannot seem to lose and keep off weight could i have some kind of thyroid problem ??? help ?

age 20 height 5,7 weight about 16 stone no medical history always healthy