There is a major milestone development at around 6 months which could be upsetting her usual bedtime routine. My advice is to go with the flow, but try to keep the routine in place as much as you can. Also, how often does she feed during the day? As she's 5.5 months, she could be starting to needing more, so maybe offer the breast more during the day?
I remember between 4-6 months was the hardest with Nina. She fed every 2 hours around the clock and wouldn't settle easily at bedtime. We just stuck with the bedtime routine that whole time, and when she turned 6 months she went back to sleeping at night again.
Good luck & I hope that helped a little.