Set backs are usually a form of maturation, but there is always the possibility of outside factors that cause them. Try to avoid putting her back in disposables--use some cloth diapers or put down a waterproof layer (lanolinized wool blanket pul pad etc.) covered with a towel under her for bed protection when she sleeps. Also make sure she's wearing clothes she can get off easily on her own, as it could be a sign of her wanting more independence.
The biggest external problems when potty training tend to be: emotional, nutritional and environmental/chemical.
First off it's getting colder, not being warm enough does influence bladder control, so another layer of clothing--especially around her belly might help her. Rainy days also tend to encourage more "misses" :-/ If your bathroom tends to be chilly, consider a space heater or moving the potty to a warm area. Baby leggings/long socks under a warm skirt/dress will allow your little girl to keep most of her clothes on while she uses the potty as well.
Has anything in her environment changed? (A new bed, different house cleaning supplies, new bedding, bedding that was just pulled out from storage . . . ) Try washing your bedding and double checking your cleaning supplies to make sure she isn't having a chemical reaction to any of them.
New foods/food reactions can also have an effect on bladder control--is your daughter truly surprised when she wets herself or is it more like "I know it's coming, but I'm too busy to take a potty break."
Exhaustion is also a huge factor at this age--you mentioned that your daughter has more trouble in the evening and night, so make an extra effort to make sure you take your daughter to the potty when you know she may be too tired to realize she has to pee--random crankiness is a pretty good sign to look for :D On days when she's been excessively active or missed a nap, try sitting her on the potty if you notice her wiggling around the bed--don't wake her up, just sit her on the potty and tell her quietly its time to pee. It may take a minute or two, but she should settle and eliminate with some gentle encouragement.
Good luck and when there's an oops take a deep breath and count to 10.