I understand how the weight can just happen sometimes. I keep my dog a pound or two underweight (since he's an agility dog), but he was taking time off from agility for a muscle strain and although he was still walking and swimming, he gained about 7 lbs before I even realized it. That 7 pounds was a BIG deal for even a big dog. So that is going to be an even bigger deal for your corgi. Getting the weight off will help immensely!!
ADCH Mikko (USDAA) SCH-B; (NADAC) EAC, EJC, TN-E, TG-E, VerO; (AKC) NAJ, CGC; (NACSW) NW1 - 9 year old GSD