Won't go down unless already asleep

[LuluHB]

My DD has gone from being a beautiful sleeper - first sign of tiredness I could put her down in her cot, she's gurgle and smile at me and then go to sleep - to not getting herself to sleep in her cot at all. She screams the place down. As soon as I pick her up she falls asleep almost immediately in my arms. After a few minutes I try to put her down again and the second she hits the mattress she wakes up screaming. I've never done CC and I don't want to start now but its starting to get really frustrating and I think she's now old enough (6 months) to be manipulating me a bit. Its taking hours to get her to bed at night, whereas before she was guaranteed to be asleep within 30 minutes of bed time.

Any ideas ladies?

ETA - Just wanted to add that she sleeps beautifully when she's actually asleep. Self settles throughout the night so I rarely have to get up to her at all, and then only if she's hungry or having teething problems.