Did you measure the uric acid content of the fluid? There have been two studies I know of that did so. One on 27 women found the fluid to be indistinguishable from urine. Another on two women claimed slightly different levels than in urine, but similar. Another study attached a catheter to the urethra of women who claimed to be able to squirt, and the fluid came through the catheter, meaning, it was produced by the urethra. In any case there isn't any established/proven/known mechanism to produce the fluid known to medicine. Claims to the contrary primarily come from people not qualified to make a statement on the matter.
It's possible that there is some explanation not known to medicine, if it does exist it is quite rare as in the cases demonstrated in the labs, in most of the studies the fluid was urine and came out of the urethra.
The upshot is that when you go to an SP and she squirts the most likely explanation is she's faking it, given that SP's fake most of their orgasms. The only way to fake it is to pee, which is pretty consistent with the "if you want me to, I can do it at will, by choice, by drinking a lot of water in advance" story the original poster was given.
If you enjoy your SP peeing on you during your session that's perfectly fine by me, just don't believe the hype.