K.W.
B.,
My daughter is only 20mos old so I am not well versed in the potty training stuff yet but I do have a reason for responding to your post. Many of the girls in my family have a hereditary condition where the ureter (the tube between the kidneys and bladder) is positioned too high on the bladder. This makes the sensation to "go" much more delayed because it doesn't kick in until the bladder is too full. Anyway, I had this problem, it didn't become evident until I was about 4, my mom had the same problem too so she was on the lookout but I think the main problem was that I suddenly started wetting the bed, would go long periods of time without "going" and run a fever. It was all fixed by a relatively simple surgery when I was 5 and the only outstanding issue is that I was warned I could be prone to a lot of Urinary Tract Infections. One of my friends in gradeschool had the same surgery too.
All that to say, if she keeps holding it, perhaps you should discuss with your doctor, just in case. Most likely it's just a weird phase she's going through and hopefully some of the other moms will have similar situations to share with you. Just wanted to bring the medical side of it to your attention. Best of luck!