What would cause a dog to go into heat more than normal(more than 2x a year)?

My parents have a shih-tzu. She's almost three, they have never bred her but have never gotten around to getting her spayed(mainly due to financial issues-my father's health has been declining and he's had major issues*being diagnosed with cancer and diabetes in the past two years*-they never neglected her though, if she needed to see a vet, they took her to the vet) they were planning to get her spayed in June, they still are-but it's been delayed since the dog just started her heat cycle.This poor dog goes into heat more than 2x a year. She goes into heat 3-4 times year. What could cause this? Any ideas?I know she came from a BYB(dog was originally my younger brothers-the 'breeder' told my younger brother to bring the dog back during her first heat cycle if he wanted to breed her).They have brought it up to the vet they had been taking her to since she was a puppy-multiple times, but the vet never seemed concerned about it.

I would suspect that her hormones are out of whack. Spaying will definitely be the best thing for this dog.

The female's hormonal cycle may be from 3 1/2 to 13 months long and still be considered normal. More important than length of cycle is a consistent length of cycle. As long as she isn't fluctuating wildly, such as having eight months between heats then four months between heats, then there is no concern.