I have been on a fast for 2 weeks and this past week, I broke out in a rash under my arms (above my armpits) and I don't know what to do? It itches!

Likely yeast. Yeast (candidiasis ) is probably the most common. Other possibilities: acantosis nigricans ( pigmentation brownish dark, not red), hidradenitis suppurativa or abscesses from the sweat glands of the axilla, contact dermatitis from deodorant etc. Doubt related to fast.
Yeast? It could be yeast from increased seating .Topical micatin (miconazole) cream would safely treat a yeast infection. You could even combine it with Hydrocortisone cream to decrease inflammation. If it persists or gets worse see doctor. It may be totally unrelated to the fast.