What Causes Itchy Rashes Under The Armpits And Inner Thighs?

I have a rash on my inner thighs about half way down also under my armpits and slightly on neck and back. It itches so bad. It is just really red just under the skin and there are some little circular white place on the legs. I have taken benadryl and used lotions and creams not helping. It seems to get worse when I get hot. can you please help?