It could be colic. My DD had colic for 5 months (she's 6.5 months now) and she was always screaming and crying about something. During the day was OK if you held her- but she could NEVER be put down.. very unsettled. She pulled her legs up to her tummy... and other times she would stretch her legs out and arch her back. I was going crazy!! I didn't take her to the hospital or anything, as I just googled colic and she pretty much had all the symptoms.

Good luck, I hope things get better for you soon!