Using a breast pump/establish proper supply

My LO is 8 weeks old and we've been down a bumpy road as far as breast feeding goes which I think has killed my supply or never gave my body a chance to get to full supply. Unfortunatly we found out the hard way, when she was just 2 weeks old, that she has terrible allergies to oats, corn and a bunch of other grains. Lactation had told me to eat oatmeal to boost my supply and that sent her over the edge as far as vomiting. One very long ER visit later we determined she has terrible allergies. We ended up having to give her formula while all of the allergens filtered out of my system, which took a little over a week. Currently we are combination feeding she takes about 16-20oz of formula and with BF for around 3 hours total per day. I'd like for her to go back to breast milk since she still spits up a lot when we give her formula but not when she breast feeds. I've been trying to pump to biuld up a good supply and stash for when I go back to work but I only get about 2oz or less every time I pump with my Spectra S2. I feel like I must be using my pump wrong or something with only being able to get that little. I cannot consume fenugreek, oats, wheat, or corn. Any advice would be greatly appreciated.