Check at your health food store and see if they sell mineral salt deodorant sticks or sprays. These kill the germs that cause odors and do not contain the toxic chemicals of most commercial antiperspirants. It sounds like your pits are the site of some serious detox. Perhaps some dry skin brushing done all over your body will activate your lymphatic system and draw the toxins aways from the armpit area. Try to avoid too much scrubbing that might irritate the rash and prevent healing.
If you don't see relief soon, you may need to be tested for a fungal infection, but hopefully this is just detox; some folks get giant zits, some lose hair, sadly you get pit rash.