Help with sudden drop in pumping output.

Looking for some advice. My baby is 3 months old. I went back to work when he was 7 weeks old. I work retail, but am able to pump at work on average every 3 hours. He had nothing but breast milk until late last week when we had to start some formula bc my output dropped a lot suddenly. I went from pumping a combined total of 4-5 ounces every 3 or 4 hours at work to only pumping 2 ounces. I know stress and lack of sleep can cause a drop in out put. We are praying I'm able to pump enough again once the holiday stress of my job gets better. I take breastfeeding supplements everyday along with plenty of water and try to eat as much oatmeal and oat products as I can. My son also refuses to nurse anymore it's been about a month of just pumping and bottle feeding.Has anyone had a similar issue? Any advice to be able to pump more? I use a madela pump and pump every 2 hours while I'm at home but at work it's very hard to get to pump every 3 hours so I know 2 hours is impossible.