“Why do I have a petechial rash?”

There are a lot of reasons to have petechiae - especially over a dependent area of the body. I would recommend seeing a board certified dermatologist for a workup - bring your labs that you have had so if there are any duplicates you don't need to repeat. Pigmented purpuric dermatosis is fairly common, but you really need someone to look at your skin in person and ask a number of questions before concluding it is a benign entity, such as PPD, especially with the geographic pattern, which is not really common for PPD.
Hope this helps.

JJC