Why Does My Dog Have Swollen Paws and Other Symptoms.?

My female dog (part Australian Cattle Dog, not sure what else is in her) has been displaying some odd symptoms lately, the most recent of which is that her back feet are swollen. First, she was vomiting and having diarrhea. Since we had recently changed her diet for variety, we thought this was the cause and switched her back (from Pedigree to Beneful). The symptoms seemed to get better, so we were cautiously relieved. Then it seemed that her spine, strangely enough, seemed to be protruding more, despite the fact that she is a somewhat overweight dog. Her belly also seemed bloated. We noticed this only the other day. Now today, her back feet seem swollen. We are making an appointment with our vet as soon as his office opens, but I would appreciate some additional feedback on what her problem might be. We are very worried and afraid it might be serious. After researching the Internet a little, I don't see any condition which would include the swollen paws. Most sites list allergies or damage from jumping, but this includes only her back feet. It seems too coincidental that her swollen feet don't have anything to do with her other symptoms. Thank you for reading this and hopefully offering feedback.