The only thing I can think of, and perhaps you've tried this already, is by trying to set up specific times of the day to remind her to go. For example first thing in the morning, after lunch, then before bed at the very least. The reward system is also a very good idea. Sometimes when a medical issue arises, regression ensues and it is indeed frustrating. Stickers can be a cheap way to reinforce positive behavior and utilizing a behavior chart might help to for her to have a visual of her own progress. I created one for my daughter and had her put the stickers on each day she went in the potty. I let her pick out the stickers.
I'm not an advocate of pullups whatsoever as I think they really inhibit the potty training process. I commend you with following through however and it doesn't mean you've done anything wrong either but based on what you're saying, it doesn't appear to be a control issue with her, it's possibly just her falling back a few steps after a medical problem. I suspect she'll come back around.
Should she continue to have problems after trying the reward system and/or reminding her to go at certain times of the day, then I would maybe revisit the doctor or even a specialist if you don't think your pediatrician is looking into it enough.
I wish you and your family well!
K.