Did some Google research as don't want to see my GP without having at least some vague idea of what she could talk about. So what I find out on Google is that the problem I'm having is called Neurogenic Bladder. It's normally classified as a "complication" from SCI/TBI and everything in between (neurologically speaking).

Wikipedia has some basic explanation about it. There are also some medical articles onlines about studies on this subject, covering it in more detail.
Let's see what the GP will tell me later on today about it.

Mark in Idaho: dehydration is not the case I'm afraid. Since I noticed the issue I have been drinking plenty of water (2 litres plus a day) to try and help things getting back to normal... Didn't see any improvement on the fact that I don't feel the need to go to the toilet, doesn't matter how long has passed since the last time. Like I said before, very strange feeling. There's no pain or discomfort though.