Milk Drying Up?

Hey mamas! I have been exclusively breastfeeding and pumping and my baby is now 4.5 months. I went back to work (new job too so it’s extra stressful) and it’s full time. The baby goes to a nanny share 8:30-5:30 during the week and I bring pumped milk in bottles, plus frozen milk to supplement. When she was born I had a strong supply and was able to freeze a lot of milk (like 75 bags I’d estimate), but we’ve been dipping into that stash more and more and I’m producing less and less. At this rate, I’m worried about having enough until she’s 6 months and that would be my goal (my original goal was 1 year but I think that’s looking impossible). I used to read posts where women would ask how to increase their supply, but is this ‘normal’? Is there a milk slump at any point and then you get over it and produce more? I also just got my period for the first time last week and feel really disappointed in myself. Any advice? Thank you