High platelet count, chronic pelvic pain as history. i found a pea sized lump under my armpit. its hard, slightly tender, and does not move or have a head of puss. the skin around it is red. what could it be? i switched deodorants but im not sure?

How high is the platelet count? Why was the count done? What you described in you armpit suggests an infected hair follicle. Warm compress may help in bringing it to a head and draining, Keep the area meticulously clean to prevent other hair follicles from getting infected. If it does not resolve in 3-5 days, it would be prudent to see your doctor. Wish you good health!