Breastfeeding

So I've been breastfeeding my baby for four weeks, whilst feeding him straight from the breast he screams, pushes himself away and falls asleep on it, leaving him constantly hungry and unsatisfied. At first we thought it was just his reflux and colic, one day my mum had him so I could get sleep, I gave her bottles of expressed milk and she said he was settled the whole time so I decided to express and give it him from a bottle which worked amazingly but now I'm struggling to produce enough milk to fill a bottle, anyone else had this problem? I really don't know what to do:-(