dog body blocking new puppy

Question
We have a 10 month female/spayed labradore and recently got a 5 month shih zu neutered male. For the first week they played well, but this week the lab is body blocking and throwing herself at shih tzu. Shih tzu now lays down when lab is around.  Please let me know how to correct this and stop the body blocking

Answer
I need a little more description, if you wouldn't mind.  Can you tell me how the Lab is doing this by describing her body language more precisely?  While she's doing this, what are her eyes, mouth ears and tail doing?  Does she make contact with your little pup, or just keep her from moving about?
A small Shih Tzu is actually smart to lie down when approached by a larger, more powerful dog, and there is even a term for it - we often call it the "small dog freeze" and it's designed to get the larger dog to move away a bit, or at least not paw at him.  A five month old puppy, if it has not already had a lot of experience playing off leash with many different dogs could be expected to be fearful, but you say that they played well at first.  Incidentally, it is often useful, when one dog has a more rambunctious play style than another, to pull the rambunctious dog away for a second or two.  If the less rambunctious dog tries to make contact again, they are probably ok - if the quieter dog tries to remain stationary, lie down or get away, you have a different scenario.
I usually don't advise people to get another puppy until their first one is fully trained.  But, now that you are already in the situation, my recommendation is to take each dog to a separate training class with a positive, or clicker, trainer in your area.  Establishing control over each dog helps you establish control over their interactions.  For example, once you are able to teach "leave it" or "stay" you can ask your dog, in a way she understands, to do those things.  Until then, supervise all their interactions, be sure to exercise your Lab sufficiently so that she won't be so inclined to pester the pup, and give the pup a way to get away from the larger dog if he wants to.