About 2 wks ago, our cat got a steroid shot for his flea allergies. It helped a lot, but now he has a cyst at the site. Should I be worried?

Call the vet and ask. It is likely a local inflammation from the shot, but take a picture, call the vet on Monday, and ask. They will probably want to see the cyst, but often a picture (if it's clear) is a good way to start.

The best answer would be to take him to the vet. Good luck and best wishes

wellness check with your vet

take him back to the vet immediately. This is always a potential problem with steroids, and must be treated

Yes...go see the vet and have it excised or at least drained. Sooner the better.