My five year old shih tsu has not been in season for two years she is very well in her self is this a serious problem or is it something that happens?

This is something you will need to discuss with your vet. She could have hormone problems or she could be having silent seasons. Some b*tches will come into heat, be able to mate and produce puppies but show no outwards signs of being in season... It is not all that common but it is a possibility.

Has she been acting any differently since this started happening?

Thank you for your reply that is interesting to know that. She seems fine in every way eats well she has three walks a day and runs after her ball really fast loves to run with other dogs and sleeps well at night she is very loving and alert dog!