Do you have a single handle or a two handle valve?The purpose of a pressure balanced valve is to maintain the same flow of hot or cold water to the valve when someone uses a fixture elsewhere in the house ,thus stoping a sudden burst of hot or cold water.If this happens each time you use the shower you could have a bad valve or you could have a pressure balanced valve now and it is not working right .It’s a little hard to say without actually being there to see what type of valve you have.