Hi Emma
I have/had the same situation with my daughter. However she does have silent reflux which we later found out, before when she was lying down, the breastmilk would come up (even though we kept her upright after feed, burped etc), this caused a burning sensation and she would end up crying and crying.
I also have the fisher price swing, and she sat in there too for some time becuase it helped keep the acid down.
However Zantac has helped, and also feeding her on an angle and I have elevated her cot.
I also feed her in a pattern, I feed her from one boob, hold her upright, and then let her lying on the rocker upright and play with her, and then feed her again, this gives her a little relief. Granted no relief to me .