Re: Rash under armpit wont go proper annoying now!!
Unexplained rashes that don't respond to treatment are sometimes caused by a bacterial infection (and/coinfections) that are caused by Borrelia burgdorferi. If it is indeed b burgdoferi, antibiotics would be needed to treat it. Left untreated, other symptoms may develop including jaw problems, knee problems, bell's palsy, brain fog, you name it.
If you live in the UK, you might want to investigate the possibility as these infections seem to be very common there.