You need to get her checked by an ultrusound on her pylorus muscle it is in the stomach.

The situation MIGHT be Pyloric Stenosis-my son had it, (it is more common in boys) the symptoms are very similar to what you describe-even the age-our son was 12 weeks it required a small operation -get it checked just for peace of mind! Immediateley-most doctors (GPs ) don't know of this you have to ask for the scan!
Her belly is big and hard because it is not passing through to her nappy /diaper-get this checked ASAP!

READ THIS!
http://en.wikipedia.org/wiki/Pyloric_stenosis