If the improvements in symptoms occurred within a day or two of starting clindamycin, then it's unlikely to be the antibacterial effects of this drug that helped you, because these take longer to manifest.
And since the improvements disappeared as soon as you stopped this antibiotic, this shows that it could not have been due to an antibacterial effect, because it would take a while for the killed bacteria to regrow.
A more likely explanation for the benefits you experienced from clindamycin would be this drug's anti-inflammatory effects.
This study on acne vulgaris shows that clindamycin inhibits the inflammatory cytokines IL-1beta, TNF-alpha and IL-6. All three of these cytokines are classes as sickness behavior cytokines, which Michael VanElzakker posits are causing the symptoms of ME/CFS.