Supply drop, need help to nurse or continue pumping.

My baby girl is 1 month and almost 3 weeks old. I can't nurse her anymore because when I do she spits up after each time. We experimented with giving only bottle and the spitting up barely happens or it doesn't at all anymore. So now I'm exclusively pumping. I'm not sure if it's my period that came back that made my supply drop or because I stopped nursing her and pumping together and started only pumping. But my supply dropped by 4oz. I'm trying to get the normal amount of water a person needs because I hate water but drinking it To make sure supply doesn't completely diminish. I'm just not sure what else I can do that will help me either boost my supply to continue pumping or try to get her back to nurse. Any advice would be greatly appreciated!!