Well, can you recall a time in your life where you noticed a woman who needed to answer the call of nature? When it comes to brainstorming new ideas for drawing or writing, I think the best thing you can do is try to recall a time in your life where you could vicariously sense something in another person that you may or may not experience yourself as a good starting point not just for creating a scenario, but also the character itself. You try to remind yourself of things like what was she wearing? What were her physical features? Where was she? What was she drinking that resulted in her bladder becoming full? And finally, do you have any recollections of the end result of her predicament?

These are some good ideas!

Of course you know my train of though... There was a mare in a pasture (or a doe in the forest - or a husky in the barnyard). She was spending time with her friends when she realized that she would have to pee soon.... Yeah, those girls and their sexy potty time! I'm sure some ideas will come to you soon. A lot of times, ideas come to me when either I least expect it or my mind is on something else.

What about someone needing to pee but they're null?