fur loss, do I need a vet

I have two cats that are 3 years old. One is a long hair (Dos), the other short(Uno). The long hair has had a substantial amount of hair fall out on his tail, near the base, and on his belly, near his bottom. He seems normal otherwise, and I saw much of the hair come out in a hairball.
The only thing I can think of is that I used Frontline on the two cats about a month ago. the short hair is normal, but maybe the long hair has irritated skin. Do I need to worry? He eats and behaves normally otherwise. This has not happened before and I have had them almost 3 years.

Firstly, a month ago means you could have fleas now, which could be why a cat would be losing hair.

A cat losing its hair isn't normal, so yes you should be worried. Go to the vet ASAP.

This could be a wide range of problems, including ringworm which is highly contagious. Definitely take him to the vet Monday.

Let us know what you find out.

Good luck!
Rachel