i have a shih tzu bitch who is three and a half weeks pregnant and i noticed today that she has started belleding (only slightly) but it has now stopped should i be worried about this (has she lost her puppies)

If in doubt seek vets advice.
I have had this happen in my breed. Someone told me that a bitch will expel pups if she is having too many. The bitch still went on to whelp 8 pups.
But if in doubt always seek veterinary advice. Its better to be safe than sorry.
Nutkin