Have the same. It is because the neural pathways / signals close down and the then it can be difficult for the brain to get the urethra to relax.
Gabapentin works on CNS by slowing it down. So it slows the nerves of your bladder to the point you have noticed discomfort of some sort. Maybe you need to change Neurontin to another med, I know that Tegretol works pretty well for neuralgias.
Make an appointment to see your doctor to discuss this problem with him.
If he has not prescribed for you to increase the medication, then it really is not wise to increase it without his supervision.
It may be that this could be a side effect from increasing the dose. You doctor will be able to advise you with regard to this.
If you are having a problem with trying to pee, take a urine sample with you when you see your doctor.
If your bladder has no urine in it, then you will not have any pee to come out, so pushing to pee is not a good idea if you do not have an urge to go.
You are doing the right thing with regard to keep drinking the water, but do not drink to much in one go and don't drink more than 2 litres in one day without the advice from your doctor. Drinking too much water can be just as dangerous as being dehydrated.
Too much salt and sugar in your diet can also retain fluid.
Your doctor may also wish to check your kidney function and your blood pressure. It may be that you may require a diuretic - don't treat yourself at home, do make an appointment and find out the cause of the fluid retention. Sometimes fluid retention can be hormonal, relating to the heart or the kidneys.
Natural diuretics are coffee, celery, cucumber (without salt) dandelion coffee.
If your legs are retaining the fluid, raise them up when you are resting.