Hi. Sounds like your due for a check up. While it could be something else (could be something like prostate in an un-neutered male or another gastrointestinal issue causing irritation), one of the most likely explanations is that your pups anal glands are impacted/infected and now needs treatment. Was/is your pup scooting? That usually indicates full anal glands that they are trying to express themselves. They have glands back there they used before they were domesticated and while they no longer use them they remain. Most small dogs and many larger need them regularly expressed or they become too full and infected. Your vet can do this for you and most groomer's provide it with their services (usually included with bath/ haircut, ears, and nails which is useful because it smells ad provides some additional services for the $) though should still be checked out by your vet on a regular basis. You should see your vet this time as infection is possible and likely and it may have another cause. Hope your pup feels better soon.