Advice for low supply

My baby just turned 4 months old. I have always produced enough to feed her, and built a little freezer supply up... Until a few weeks ago. I was diagnosed with stomach ulcers after struggling with excruciating stomach pain for a couple weeks. I have been on Prilosec for the past 2 weeks now, and have noticed that my supply is dwindling. I used to be able to easily pump 5 oz in the morning while my baby is still sleeping, and now it's a struggle... sometimes I only get 3 oz. Then I nurse her around 10am and she still acts hungry. By the time she is ready to eat again (around 2 or 3pm) I don't feel like I'm full yet and she nurses and then I give her a bottle. That pattern continues all day. She pulls off and cries and acts ravenous so I know she is still hungry. I am also hypothyroid and and taking levothyroxine, and just finished my first pack of b/c pills since having baby. I have an appointment with my midwives this week but I just wanted to know if anyone had any advice... I am terrified my supply is diminishing and I won't be able to bf much longer.