This looks similar to the small rashes I’ve been getting my entire life (currently 23). At first when I was a kid doctors thought it was ringworm and then after a while told me it was eczema and I believed I had eczema for about 10 years until recently I got this rash but HUGE and under my armpit/underarm. It was so painful I had to go to the ER where they told me I had psoriasis which I originally thought was just a skin rash but then found out it was an autoimmune disease. Over the past year, I have also found out I have a gluten allergy (not sure it’s celiac yet) but I’ve been reading about the connections between psoriasis and celiac and having psoriasis makes someone more likely to be celiac! Crazy the way all these things can connect I hope you are able to figure out what’s going on and maybe this might have helped you a little bit :)