Yes, I’ve been caught out by this phenomenon many times. It’s definitely a thing. I’ll have a day or day and a half of feeling oddly well, start up some long neglected projects, get in touch with friends, chatty and brain working, make arrangements to do stuff and meet up... then, wham! I’m crashing with viral symptoms feeling as bad as ever. It’s really inconvenient and took me many years to suss out! It’s not simply push-crash, even keeping the reins on it happens.