In Becky Chambers’ Wayfarer series, there is a species who actually breathes methane. The focus though is less on how that actually happens and more on how they navigate as the only species for whom oxygen is toxic. It’s a great series, btw. It’s a not-quite-as-optimistic as star trek future, but still optimistic and with a vast range of species who are all intermingling as learning how to get along.
I don’t know what country you’re in, so something specific can be difficult to say as to where to go. It can often take a few days (3ish sometimes) for some tests to show positive, but get tested and “test where you play”. It should be cheap or free. It probably isn’t a big deal, so don’t freak out, just get tested. Go to a clinic or a doctor’s office, or call them first.