If you don't even know about their heat cycle, you have no buisiness breeding. Get the dog Spayed.
I must say that I have a passion for Shih Tzu's. I have a beautiful female that I got at the Animal Shelter, Yes even pure breeds end up in Animal Shelters. I also have a very poorly bred Shih Tzu that I saved from a lady that wasn't taking care of him. He has an Underbite, his eyes are wonky, he had cryptorchidism (only one testicle decended) ,he seems to have breathing problems too, and he does not look like a Shih Tzu should. I wouldn't be surprised if he has more health problems in the future. He is a sweet dog, but just because you have a dog that is in tact doesn't mean you should breed them. You need to get some knowlege about breeding and do testing on your dogs or your dogs could turn out like this dog.