Can I just add to this comment that
@Claire W is treating her guinea pig for an ongoing long term issue with regular flare ups, as this is one; she has been given medication for just that case.
As a forum, we emphatically do NOT support treating your piggies at home on spec unless absolutely necessary in case you really cannot reach any open vet clinic. UK vets are increasingly in the process, or have done so already, of setting up a video facility to help with judging which cases need to be seen and treated in a hands-on consultation.