I got woken up this morning by my GF asking for paper towels and cleaner\ , and I asked why
but not only did it wake me up,
So which is it... That you were awoken to getting wet, or by your girlfriend?
If this story is Real, then either she has a medical condition...
wanted to introduce her watersports fetish gradually to you,
or it wasnt pee, but feminine juices.
Or it was an accident. Dont think Ive ever heard of such an accident at such an age however.
If its medical, or she is unsure, then she should get checked out.
If its watersports, then tell her 'In the bathtub only'.

heh
If its the last, then wow...
In almost all cases above, the purchase of rubber sheets may be in order. heh
Best way to handle this, is actually discussing it, and finding out what is really going on.