Breast milk supply drop at 9 months?!

Hi all! I have been breastfeeding my little baby for 9 months and pumping on days I work... I was an over-supplier and I would pump 20oz before going to work in the morning and I actually donated 11 gallons... yes GALLONS to a milk bank... but now ever since my period started back at 7 months, I noticed my supply drop steadily, particularly really low right before my period. I am 5 days late on my period now (not pregnant, took a test!) and I just feel like my baby isn’t getting enough!! I had to warm up my freezer stash to top her off... I was hoping to nurse her for a year but now I’m barely getting 3.5 ounces!! HELP!! She has been eating solids great so that could be part of the reason. How do you keep your supply when your babies start solids?!