http://onlinelibrary.wiley.com/doi/10.1111/jsm.12799/epdf
Conclusion: "The present data based on ultrasonographic bladder monitoring and biochemical analyses indicate that squirting is essentially the involuntary emission of urine during sexual activity, although a marginal contribution of prostatic secretions to the emitted fluid often exists."
This is recent and uses only seven women. I agree that that number is low, but to suggest that their experience doesn't represent other women squirting is to suggest that they're just faking it or something? I mean, if this is the best study we have then it simply makes more sense to trust it than to believe whatever we would prefer to believe. I agree that your experience could contribute to the knowledge we have, if it were tested scientifically. Unfortunately, what you have now is a story because without proper testing you don't know where the fluid was in your body or its chemical composition once it came out.
The study showed the bladder filling up and then emptying during the process, and showed that the fluid was almost all chemically urine. I can't comment on your personal experience except to say that it doesn't invalidate the scientific findings. I don't think that the fluid being mostly urine takes away from the experience at all, but it isn't amniotic fluid. Amniotic fluid is produced during pregnancy and is stored in the body with the fetus in the amniotic sac. These tests show that the liquid involved in squirting is stored in and released from the bladder and is chemically mostly urine.
Like I said, maybe the tiny amounts of other fluid with the urine change it in some drastic way, or maybe the subjective experience is altered for all parties involved by arousal, or maybe some combination of those things is at work. All we know for sure is that it seems to be mostly urine, which doesn't take anything away from the experience of course. Amniotic fluid, which you mentioned, starts out as mostly water with electrolytes and nutrients develop in it over time, and it also contains fetal urine eventually.
"Urea, creatinine, uric acid, and prostatic-specific antigen (PSA) concentrations were assessed in urinary samples before sexual stimulation (BSU) and after squirting (ASU), and squirting sample itself (S)."
The main difference they found was that in the sample before squirting there was no prostatic-specific antigen ("PSA:"an enzyme secreted by the prostate gland) found, but in the samples of the fluid from squirting itself and in the urine sample taken afterward, the PSA was detected in small amounts. That enzyme was the noticeable difference they discovered because it was the only thing not present in the urine sample before squirting.