Small red rash on inner right thigh and knee, it burns when shower water hits it, what can it be? they look like tiny red dots, no recent changes in diet or detergent; i took a benadryl (diphenhydramine) just in case

Skin rash: It is difficult to diagnose in your case . It could be nummular dermatitis, eczema, psoriasis, contact dermatitis, atopic , or another eczematous condition. Using emollients and a corticosteroid will be helpful.

I would say: Most likely in tina corporus. This is like a fungal infection. That not necessarilly it but look it up and try an otc antifungal. Will need it for 3-4 weeks as it stops growth as skin turnover removes it. If it itches badly you can mix lotrimin (clotrimazole) cream with Hydrocortisone cream if is in fact a fungal infection. Good luck.

Hard to say: Could be early shingles. Have you had shingles in the past?

Mystery rash: It will probably remain a mystery and be gone before you finish the week. There is no way to tell.