Rashes are tough
Rashes are tough to diagnose without actually seeing them. However, if by "circular rash
" you mean a circle-shaped rash with rash only on the edge and clear in the middle, this could be ring-worm, which is a fungal infection
in the skin (not a worm).
The bumps on your rump could also be a fungal rash. Advise that you see your doctor for an examination to determine the cause of the rash and treat.
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Answer assisted by David Miller, Medical Student