Had spot on lower left leg 5 days ago. Diagnosed as cellulitis. Started clindamycin 150 mg 4 days ago. Pain gone but foot really swollen in evening?

Reevaluate. Start over. You have failed to give enough information for an intelligent answer. Only general observations are possible. A diagnosis of "cellulitis" by anybody not a dermatologist is wrong at least half the time. Clindamycin, because of a risk of C. Diff.,should seldom be used. See a DR, perhaps a different one, to consider an alternative diagnosis. It is likely that your problem is not infectiio.