Is your mom feeding her anything else besides breastmilk?
I'd suggest that you spend the weekend in bed with your baby, nursing. Literally. Other folks will be able to suggest herbs or other substances to increase milk supply (oatmeal is one I hear mentioned frequently, in addition to fenugreek). But nursing more frequently, for longer periods of time, is the primo number one way to increase your supply.
If your mom or her presence is stressing you out, then maybe you could gently tell her that your milk supply is down, and that you really need to spend the weekend and the days thereafter getting it back up to par, and that you'd really, really appreciate it if you could have a bit of space in which to do this. Dunno how feasible this is (eg, is your mom in from out of town, and is staying with you for a fixed period of time?). But it's worth a shot.
Good luck! By the by, you might want to check out the breastfeeding board ("Getting started and overcoming difficulties") re questions about how to increase supply. You might also want to run a search in that forum for prior discussions.