Although bitches normally go into heat every six months, there is some variation...I had one that was on a five month cycle for instance. Your dogs humping when your bitch is not in heat is just dominance play and not sexual.
One of my Chins had an allergic reaction to the vaccines as well and had to be rushed back to the vets. Now he goes in a few hours before his vaccine and they give him an antihistamine shot before his vaccination and they keep him for an hour or so afterwards to make sure he has no reaction. He hasn't had any problems since.
I don't have Shih Tzu's but (from what I understand) their dietary requirements are very similar to the Chins and I feed them a combination of Royal Canin and home made food and supplements. Since the dog food scare they get far less commercially prepared dog food than they do food that I prepare myself, they seem to like the home made stuff far better and I have the comfort of knowing they aren't getting anything that would make them sick. Your female will need high quality food before, during and after any breeding. There are many excellent books on canine nutrition and all sorts of informative articles online, but the best place to start would be the breeder(s) that sold you your dogs to begin with. They should know what is best for your dogs and be happy to share that with you. | 
Any Shih Tzu Owners Out There? 
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