If you look at my post to ForgetMeKnot, in this same board, you will find a possible explanation for your urine like feeling:
There are a large amount of literature about the G-Spot you can read. In the book A New View of a Woman's Body by The Federation of Feminist Women's Health Centers, there is a very clear information on this matter. You can get this book for about US $ 20 in bookstores or thru the web. Also in the web you can find a large amount of information regardles of all the nasty thing you will find you has to avoid.
Also in this book there is a good explanation of how female ejaculation occurs. This fenomena is extremely pleasent for women as well for men when both learn hows it works. Is not an easy activity at begining but once you learn will give you a huge amount of satisfaction.
With regard to the location, you can find it in the upper wall of your vagina toward your narver and about 2 1/2" inside the entrace. If you partner or you stimulate this area you will feel it changes in texture and also will feel some pee like need. If you stay relaxed and let you got you will probably ejaculate. If you do it it will not be urine, just another liquid which comes from some sponge like tissue located between your bladder and your vagina.
BTW there is no possibility of urinating while you are argasming. Urine destroy spermatozoids, so you cannot urinate, since spermatozoid has more importance for reproduction than urine, this urge is avoided for your body. Now as the ejaculation comes thru the urethra you fell pee like sensation, but as said is no urine