She definitely doesn't look thin. On the contrary,(whispering) she looks a "pleasantly plump" BUT, I do know it could be just the picture.
Regardless of what her weight is usually at, mysterious weightloss and/or appetite loss is cause for concern.
How long has this problem been going on?
How old was she when she was spayed?
Did your vet say what the reason was for the inflamed gum? Dental disease in rabbits can be the cause and there are several different issues that can cause dental disease. Here is a link about it www.veterinarypartner.com/Content.plx
FYI: If the runny stools was happening while she was on antibiotics, then make sure to tell your vet. As antibiotics can actually cause this. Some vets will try a lower dose or switch to another kind.