I had the same problem with Emily.The low iron formula made no difference.The best thing was Kiwi fruit.We got senna from consultant at hospital in the end.Also try vaseline around bottom area as it eases the poo out.
Also she'd cried constantly and I got to the point where I couldn't cope much longer.Although it took me about 4 months to admit that.Things got steadily worse,she would screech so much as if she was in pain.
I once saw a BW program with Tracy getting down to babys' level and speaking gently and telling baby where you were going ie;kitchen,bathroom etc.I didn't remember any of this until my health visitor said about it.Also what helped was taking her to the kitchin with me and instead of pushing her away when I was microwaving milk or waiting for the kettle to boil just put her up on the side and showing her what was happening.
I got so bad with the stress of it all I finally admitted it when I found myself at the top of a cliff on holiday thinking this would make her shut up.
I had no intention of doing anything I just said it and walked on.I then phoned HV and told her to take Emily away as I can't cope with the crying,pulling at me,scratching etc.
Although she started this at about 6 months it did take me to 10 months to get help
Get your health visitor in to chat.Mine was great.I don't know if I've been any help but you are not alone
Also have a look through the BW books if you haven't already, they are great help