No, it's not too early for cats to have kittens. They are right on schedule.
My vet told me once that length of day starts to trigger a heat cycle in cats and some cats will continually come into heat every few days until they are bred. In my own experience here on the farm, cats can and do interbreed (doesn't make for healthy kittens) but it happens; although they don't always interbreed. Several generations of inbred cats makes for and inbred 'look', but here I get my cats neutered/spayed.
I medicate for worms and any illness and they get fed regularly. We don't have many rats in the barns, none around the house. My cats don't leave our farm, they don't need to. They're fed and neutered, no need to roam. Cats will also have litters of kittens late in the fall, which have a lesser chance of surviving because of the cold. Some mom cats will have 2 litters of kittens a summer, and a lot of times will abandon the second litter because of the toll it takes on her to provide milk and attention for them. I guess for that second litter, she's just not into it anymore.
If idiots would stop making their problems MY problems by dropping off pregnant females and litters of kittens--I wouldn't have to own stock in my local vet.
The shelters won't even take cats/kittens anymore. They also drop off litters of puppies on the less traveled back road. Pathetic.