p.s. If you "suddenly blow up with excess fluid" fundal height will not be the first indicator, excess fluid is marked by swelling - mainly round the face, the fingers and feet and also on its own is not a problem, excess fluid is only a problem when present with high BP and +++ protein in the urin
You are talking about GD. Polyhydramnios doesn't always have these symptoms and and can indicate problems with the baby, or indicate other risks such as early labour.