It is possible it's the start of PUPPPs if there are a few lumps because stretch marks are generally flat or slightly indented or you might be lucky & it's a heat rash. Try putting some cream on it, because it will get worse if it dries out, sorbolene or an aloe vera cream will help. Just whatever you do, don't use calomine lotion because it will dry it out & make it 10 times worse

Post again in a couple of days how your going because as I said I have lots of things to try if it is PUPPPs. Best to get the remedies working as soon as you know because mine was out of control by the time I got onto it properly.