Clindamycin is used for (among other things), soft tissue infections, so I'd think it'd be a good choice since it sounds like Shasta's gums are infected too.
It can cause diarrhea (as most antibiotics can) so just keep an extra eye on the LB (Oh JOY!!

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Yeah, it's kind of a normal side effect of the AB's since they kill the good bacteria as well as the bad. I know a lot of people use FortiFlora probiotic to help add some of that "good bacteria" back into their gut to help with that.
Pumpkin can also help ....or baby food squash (no pumpkin pie mix or added sugar of course)