Why do I get rashes after lying on a grass patch?

Hi, I can understand your concern about the rash in view of your exposure history. Certainly, some STDs can cause rash/ skin lesions like Syphilis, Herpes and HIV. Typically, secondary syphilis (that means syphilis that has not been treated and the disease progress to 2nd stage) can cause a rash but the rash is usually not itchy and usually appear as small bumps. Herpes usually cause blister-like skin lesions. HIV rash can vary in appearance and there is no specific characteristic rash that can definitely indicate that it is a HIV rash.

Thanks for the question. Pityriasis rosea is a condition that typically presents with scattered red rashes on the trunk and arms and thighs which may be scaly. The face is often spared in this condition. There is a herald patch, often a larger patch that appears before the rest of the rashes, but this is not seen in all cases of pityriasis rosea. The face is also a very unusual site to get the herald patch. So overall, it is really difficult to comment if your rash is truly pityriasis rosea without seeing your rashes.