A girl I saw in civvie world was a squirter. Her ejaculate was quite bitter to the taste which contrasted quite significantly from her normal secretions, and indeed her piss.
See that contrasts with my experiences. I have generally found piss to have a bitter taste, but the flavour of squirt (from the two girls I have tasted) has always been slightly sweet, with no bitterness.
Two other differences I have found between squirt and piss: squirt seems to have virtually no odour after it dries, and the feeling of it on your skin after it dries is almost nil, whereas piss tends to make my skin feel a bit, well... hard to describe, but you have all wet your pants at some point and know the sort of itchy discomfort I mean, right?
Also squirt has never been yellow at all in my experience. If it's piss with a few secretions added, there would be yellow colour to it.
Having said that, the squirting I have experienced was fairly small volumes, not huge quantities. If there's a lot, then yes, the bladder is the only explanation of where it could come from.