Looks like a reaction to something. Are you wearing anything metallic next to your skin? Could it be a reaction to a bite? You could try taking an anti histamine tablet one per day,but if that doesnt help, or it spreads then time to see your doctor.
I have got a similar looking rash on my back, stomach and thighs. it started early july and has just spread and spread. The gp gave me antibiotics for a week and some cream(incase its bacterial or fungal) both of which have had no effect and its still spreading. I have a follow up video call with my gp tonight and hoping she has some idea what it is. i will attach a picture of what mine looks like now, it started small like yours and has spread out with the patches getting larger and overlapping in some places. have you spoken to a gp?
How can you tell if a rash is serious?
If you have a fever or pain accompanying the rash.
If you have a sudden spreading of bruise-like lesions.
If your rash continues unabated.
Any rash that is widespread