@dinnerlady most likely MS related. Ideally your MS team (forget the GP) can get you seen by uro-neurolgy who can do a number of investigations to find out exactly what your bladder is doing, or not doing. Thereafter there are a few medications they can prescribe which may manage this for you. These are measures that can be done before other things such as Botox and/or Intermittant catheterisation.
Oh yes, and you will be advised to avoid caffeine and fizzy drinks.