5 month old was sleeping thru the night (~12 hrs) for past 2 mos but now sleeps only 6 hrs, then wakes ~every 45 min. How can I get her back on track?

Avoid night feeding. Make sure that she is falling asleep in her crib, not during the last feeding. If she wakes up and looks around, everything is the same and she may put herself back to sleep. If she cries, go in and calm her, but don't feed her (it's not hunger) or take her out of the room. Repeat until she understands that it's sleep time. It will take a few days to get back on track.