Hi, ALO! This is a very common situation for families to go through. It's understandable that you feel overwhelmed and worried. It sounds like there are a lot of contributing factors.
Your menstrual cycle can certainly play into the drop in supply - this article may be helpful:
Solids can also impact your supply. If she's eating a lot of solids, even if she's still going to the breast as much as she always was, she might not be transferring as much milk, thus, we'd see a decrease in milk supply. Pumping every 4 hours at work may be playing into it, as well as the stress you experience at work. You didn't mention if you nursed at night or pumped during the night - if these feeds/pumps have been eliminated, that will have a negative impact on supply as well.
At the end of the day, if she's growing and happy, that's important. We do want breast milk to be the primary source of nutrition, so I would probably cut back on how much solid food she gets. However, it's hard to say without meeting with you directly the exact cause of what's going on.
I would suggest you book a consult with a member of my team to determine if there's anything else you can do at this point!