Before I had ME/CFS I had a short course (5 days) of amoxycillin for a tooth root infection. That was when I was fit and well, and walking my dog miles each day, in 2006. And amoxycillin is a pretty mild-ish antibiotic and one I can tolerate okay.
I remember having no gut issues, or in the years to follow, even though I didn't take probiotics. Well....I did. I took a grocery store mutivitamin which was supposed to contain them, but I since found out those are dead!
But I remember while on the course, having the most awful unfit fatigue, and could only walk my dog for an hour each time instead of the usual 2 hours. My legs felt like lead and I was a bit "brain-foggy".
Literally the day after I stopped taking them, my energy went back to my normal state.