Heat rash ???

:sad: Ok, I started excersizing again about a month or more ago and everytime I am done with my walk or excersize I break out all over my body with what I think is heat rash. Its small very very very itchy red bumps and welts all over me! I hate it, the only good thing is after I cool down they do go away, eventually from a half hour to an hour.... very annoying and irritating... has this happened to anyone? This has not happeded to me ever and I have done some good work outs in the past. I didn't work out all winter and I did figured maybe my body was getting rid of toxins?? I'm only guessing! HELP! Wondering what I can do to stop them!