Can't keep up

My supply can't keep up with my baby today. She is 7 weeks old and nursed this AM and then I pumped about 5 oz. every since I pumped I feel like I'm empty. She has nursed but after a few minutes you can tell she isn't getting anything so I switch sides and it is the same thing. I let her continue to suck to try and get something stimulated. I have pumped after she nurses to attempt to stimulate also and I get less than half an oz. She has a milk sensitivity so I had to go buy expensive formula to supplement with tonight. 😔 Has anyone had this happen and their supply pick back up?