Diarrhoea in 11 year old West Highland White Terrier (Westie)?

I have a Westie who's going to be 12 in September and she's got constant diarrhoea. She's on a special veterinarian diet and is up to date with her vaccinations so I don't think either of those are her problem. She constantly whines like she's in pain but we're reluctant to take her to the vet as they're expensive and she's too old to insure. She's also shivering but that might be due to the fact that she's just had her hair cut - I don't remember her shivering beforehand. Please help as we're at our wits end!