My daughter did the same thing beginning at 4-1/2 y.o. She regressed and began bad potty habits. She developed a U.T.I. and now sees a ped. urologist. From developing bad potty habits by holding it too long, she formed a thick muscle around her bladder. She literally cannot feel it when she needs to go b/c the muscle is so thick from holding it. Incidentally, she had a VCUG (x-ray w/catheter) and she has Stage 2 kidney reflux (congenital) and holding in the urine didn't help. Also, when they hold in urine, they hold in poop causing constipation and creating more likelihood of U.T.I.s. Miralax or natural remedies are good for this. Another thing that occurs with all of this, is that when the muscle is so thick, it is hard for the bladder to fully empty. She ends up needing to go more often and will leak or wait to the last moment when she's about to burst. Ditropan may help relax the bladder, but it didn't work for my daughter and certainly didn't change her potty behavior.
Basically, you need to nip this in the bud now. Go to www.pottymd.com and they have gadgets such as a vibrating watch alarm that can remind her every hour to go potty whether she needs to or not without making a scene at school. Talk to her kdg teacher and tell her she needs to allow your child to go often (and insist upon it). She has to relearn how to go and thin out her bladder muscle wall so that she can start feeling it. This takes some time to do. She needs to also sit on the potty and relax for at least 2 minutes, even when she thinks she is finished. My daughter also is unbuggable and didn't care. It was so frustrating. The only thing that worked to make my daughter understand the critical nature of this problem was that if she didn't come home from school with dry panties, she would not get t.v. that day. All she needed to do was to go potty when she needed to and she would stay dry...that's it. After a few days of no t.v. she got the picture. When she walked in the door from school dry, she was so proud and excited. A reward system didn't work, only a threat of taking away something she valued did. Whatever it takes. My daughter is almost 8 now. It has been a real struggle in our house and has been hard on my relationship with my her, asking constanting if she needs to go, blah, blah. We're still working on it. Her daytime potty habits have improved 10-fold. It has become so much more natural for her to go when she feels it. I'm sure her bladder muscle is thinning out. Unfortunately, she still wears Goodnights at night and will continue to I suspect until she's a teenager. Very hard on a child's self-esteem. Good luck to you.