Its unusual but not impossible. Usually it's about 4 months before they go back into heat I'd estimate. It could be an indication of a more serious problem though like a bleeding uterus or something. You really should take her to the vet and make extra sure theres not something more serious going on.
When I was little, we lived on a farm and our cats were never fixed(my moms doing) we had kittens all the time. We had this one cat I remember it seemed like as soon as she had her kittens and they were 3-4 weeks she was pregnant again and by the time the kittens were weaned she was ready to have more! (Just so you all know, she had 4 litters of kittens and we gave her away to someone who made her a housecat and had her spayed)