Spayed female in heat after 2yrs??

I have a female dog, about 3yrs old. I just got her from a close friend who is moving, she is a lhasa/shih tzu (mix or either..) She got the dog two yrs ago from a bad situation. She took the dog to the vet for care and was told she was already spayed. Well a few weeks ago, she went into heat. This is the first time she has ever gone into heat, in the last two yrs. So I find this very strange, if she wasnt spayed she should have went into heat previously, and if she is spayed, why is she going into heat suddenly? Any ideas out there? She is otherwise perfectly healthy and shows no signs of infection or anything. Simply in heat, going through all the proper changes for this.

There are a few possibilities that come to mind-

1) The vet mistook some other scar for a spaying scar. Is it possible your friends might not have recognized the signs of a dog in heat?

2) Sometimes other things, like some kinds of bladder infections, can have 'symptoms' similar to a heat cycle.

3) Sometimes they don't remove all ovarian tissue when spaying, either because the dog's ovaries were abnormal, or the spayer was sloppy, or whatever. So she could still have a heat cycle, though she wouldn't be able to breed succesfully.

I have heard of these happening through working with shelters/vets, but have never experienced any. Sorry I can't help more.

Forgot to add--- just to be on the safe side, I'd keep her away from intact males, and also get her checked out by the vet again. He may want to check for bladder infections, ovarian/uterine remnants, etc.

She has always had a male dog in the home with her, so even if she herself didnt notice, im sure the other dog would have. This started with slight swelling and bleeding that lasted 3-4days, bleeding tapered off and she is now very swollen and has a light clearish discharge. I just got the dog less than a week ago and she has a male St. Bernard, and he was going crazy, so this is deffinately her in season. What confuses me the most is not happening in the last 2 yrs.... I know the vet could be wrong about the scar also. Just the lack of for the last couple yrs thats strange. Maybe some tissue did get left...could that take time to *grow* enough before producing enough hormone to cause her to go into heat? (She has also taken a liking me, gets a bit bothersome at times. LOL She tugs on my clothes and licks e like crazy, then trys to hump me! Does the same to my coat, she sleeps on it, but also drags it around and trys to hump it. Hopefully she dont stay in season for much longer. This is a new behavior for her, ive known the dog for as long as my friend had her and she never done this to me until the day I went over there to get her.)

I don't know for sure, but if she is in heat due just to tissue remnants, I think it would probably be a pretty irregular thing, not as reliable as an unspayed females. I really don't know for sure, though.

I agree with Allecto. No matter what though I'd get her re-checked by a vet.

They can be like us ladies we had a mixed breed female who only came in to heat evey 3rd year so after she became 8 we figured she was done (ya a huge singlr pup in the morning one morning proved us wrong. Sadly it must have done sonething toher not long after weaning the pup she died. She was a german shorthair ret/ beagle. She looked like a whippet marked like a Gsr.
I would get her in and make sure she was spay or if she has a physical problem.