My arms itches and gets a similar sor like a pimple but it starts out as if it is a mesqeto bite then it goes yellow like a pimple ?

Folliculitis?: Could be folliculitis- infection of hair follicles. The yellow means it it likely infected and needs to be drained and treated by a Dr. with antibiotics. Talk to your Dr regarding this problem. For prevention, use a rough/mildly abrasive washcloth, loofa or brush to wash your arms and legs- it will help keep the follicles from getting plugged.