Was it antibiotics or steroids to control the allergy? If it was steroids, then that could increase water consumption and urination. Best to call the vet to find out exactly what the injection was and ask if it could be causing Scooby's problem.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."