Why the change in behaviour?

by Louise
(Herts, UK)

My female Shih Tzu Tallulah is nearly 11 months old, she has been such a well behaved pup and I've been very lucky. I have noticed in the last few weeks she has just become relentless, wanting to play ALL the time, barking a lot and generally mis-behaving. I am getting her spayed next week, not because of her behaviour obviously but I am hoping it may calm her down. When she is being really hyper, she just sits there panting the whole time.

Is this just a phase? Maybe her teenage years? Feel I am shouting at her a lot and feeling guilty. Any advice would be appreciated!