Is she waking up wet or dry? Many kids will continue to wet at night for quite a while even after they're day-trained.
My older daughter was day-trained by 2 1/2 or so, but she wet her pull-up every single night until she was 4. A week before her 4th b-day, she decided she wanted to wear underwear to bed, so we let her try. We had several setbacks (going back to diapers) and LOTS of wet beds, but after about 3-4 months, she was consistently dry at night.
My younger daughter was day-trained easily by 2, and at 2 1/2, she decided she wanted to quit wearing diapers to bed (the same time as her sister). She had already been waking up dry at least a few times per week, and she ended up staying dry more often than her older sister. She was consistently dry at night within 3-4 months as well.
My girls share a bed, so we just bought a mattress cover that goes under their bodies over the top of the mattress. I can't find an example online right now, but it was just a pad that went across the bed, over the sheets, and tucked in on both sides. It was very easy to just take it off and put a new one on if we had a middle-of-the-night accident.
Hope that helps!