Very unhappy little 5 week old baby

Thanks again all. Will join the Sep babies forum as well.
I have stopped the Ranitidine as of today as I think it actually made her worse. I will continue the lactulose for another couple of days and then see what happens. I wouldn't be worried about her not pooing if she was a happy baby, but I just have a feeling that it's all linked. The lactulose worked great to start with - she pooed 3 times in 2 days. But she now hasn't been since Thursday, so it's either not working, or there isn't any poo to come out!!
Will def try the magic hold next time she's upset.
We try to have her down within an hour/hour and a half of getting up, if she's happy. Lately she's been grizzling for the first hour to 2 hours of going down, and not falling asleep for ages. But we try not to pick her up, just go and reassure her briefly before leaving her to it. When she's in pain and screaming, she can be up for 4-5 hours at a time, until she exhausts herself completely and falls asleep on me. But then she often sleeps through her next feed because she's so tired, and that bothers me. But I never wake her.
She also farts a lot during her 'screaming, knees up, arms flailing' episodes, and they are incredibly smelly.
I side sleep her most of the time as she is happier that way.
I'm going to go back to the GP this week.
Thanks so much for all your help. It's so nice to hear there are other babies out there like this!! I keep telling her I'm going to take her back and get a new one coz we got a faulty model!!!! LOL. We just can't wait until we have a happy wee baby.