I'm a mother, grandmother, and gr.grandmother, and I have a pretty good idea of what you're talking about.
I found that eliminating solid food too close to bedtime is one way. Her bowels are on a wrong schedule. I noticed that in the time slot babies were born that it affected their sleeping schedule. If born early morning, it was very hard to get them to sleep at night, but when born at night, I noticed that they slept better at night. A lot could depend on how much she nursed at night, that could affect her bowels, and her needing to eliminate at night.
Try feeding her an early supper. say no later then 5:00pm. and not allowing her to eat passed then. There is a good chance that she will go before bedtime. also encourage her to go pooh in the toilet like a "big girl" does, and I think that if she wears pretty panties, as some have mentioned, will encourage her to change her habits, also making her clean herself, is an added incentive, because she'll realize how distastful it is, and isn't likely to want to repeat the experience. I think that, that was an excellent suggestion that some mothers had.
I don't believe in the bribe system, because it fosters some very bad habits as they get older.They will start to believe that you have to bribe them into being good, and that's a bad pattern to get into. Hope this helps. My husband is a Disabled serviceman, and my heart goes out to you at this time, and many thanks to him for serving our country willingly, as my husband did. V.