Your problem sounds so familiar. Just 10 months ago, I was going through the same thing with my co-sleeping DD who is now 21 months. The reason for the night waking could be any number of things. Perhaps she's teething or going through a developmental spurt.
At the time, I suffered the sleepless nights (she went back to nursing just two or three times a night after about a month), but now, having gently night weaned her, I would do things much differently. I started offering my DD a sippy cup of water during the night. She lightly fussed about it for two nights, and then started sleeping through. She would then nurse at night when she went to bed and again around 6:00 a.m., which was fine with me.
Since about 18 months, my DD has been sleeping from 8:00 p.m. to 6:00 a.m. solid. It's so nice to get some sleep again. Hope you are able to resolve your problem.