How to get my 12 week old baby to stay asleep!?

My beautiful little girl will be 12 weeks old on Sunday. She is suffering from colic and tends to have crying spells in the evening. My problem is that when I do eventually settle her, as soon as I put her in her cot she wakes up again, This is any time of the day. She seems to jerk and twitch a lot in her sleep which wakes her up. The only way I can get her to sleep for a long period of time is held closely against my chest. I don't mind this so much during the day but obviously it's not practical at night. Any ideas as to what may help and what may be causing the jerking? I have tried swaddling her but she always manages to wriggle herself free! Thanks.