D.M.
J.,
Good for YOU!! I am a co-sleeper with a 2 year old, and the benefits are amazing. However, when you get to this age/stage it can be tougher because the growth spurts mean more need for nutrition and also, she may be teething and the sucking helps ease the gums.
First, I would try to nurse her as long as possible without letting her fall completely asleep and see how long that takes. Usually my son would fall asleep while feeding, and then wake up in a very short time to feed again. Once, I started doing things to keep him awake while feeding the last night feeding before bed...like rubbing his cheek or just talking to him...he'd feed for a longer period and then got used to feeding until full and then only waking up when hungry, which was usually every couple of hours to three hours.
If it's teething, check her gums by rubbing on them to see if you feel any teeth breaking through...you may want to invest in some Hyland's Homeopathic Drops or tablets to keep on hand. My son started teething at four months, and broke his first tooth at five months and the drops helped him get a more restful sleep in between feeding on demand.
Don't worry, this is all totally normal. I went through a month or two were I was wondering if I was doing something wrong. We got past it, and had about a month of calm before the next phase when we started solids at around five months!!
This is a huge learning experience for you AND your little girl...just keep doing what you're doing and everything will be just fine.
Best of luck to you!!