What Causes Burning Sensation In The Armpit?

Hi i suddenly got a sharp burning senstion in and around my left armpit and it hasnt gone for 2 days, there is a slight rash but it is very painful to touch or even move my arm, i dont no if it has any connection but i had surgery on my knee appox 14 weeks ago and being investigated for artheritis, i have used the same de odourant for 10 years and have not changed shower gel\soaps i am 26 yrs old Thank u for your help