When did your milk supply regulate? Did it change all at once or gradually?

My pumping output dropped suddenly a week ago. I've been pumping since Day 1 (DS spent a week in the NICU after birth) and have always had fairly consistent outputs. I typically pump 3x a day while at work, pump for any other missed feedings when DS takes a bottle, and otherwise EBF. Since I started back at work in mid-Dec, I was pumping 450-700mL/day (about 15-23oz). Last week it suddenly dropped to 300-400mL/day (about 10-13.5oz).

I can't figure out whether this is a normal supply regulation or due to a combination of outside influences:
-DS started STTN more consistently (went from waking maybe once to not at all)
-I changed my diet slightly by cutting out most junk food. So I'm eating "better" and still eating often, but my overall calorie intake probably dropped.
-DS is getting over RSV. He started coughing the day before I noticed a change in supply, but was not very sick until several days later. At that time he had a very low appetite, but I was pumping a lot more to try to make up for it. His appetite seems to be back now.
-As of today, I seem to be catching a cold, probably from DS

Also, DS is 13 weeks and has always been <1% percentile for weight (I had to be induced early). At his sick baby ped visit, he had only gained 3 oz in 2 weeks (he was sick for a few days at that point, so let's say 3oz in 10 days). I'm always concerned he might not be getting what he needs to maintain weight gain.

Should I worry? Taking fenugreek and increasing the number of times I pump each day has not had any positive effect. I know EBF is the best way to create the optimal supply, but I obviously can't do that while working full-time. I would prefer not to call up the LC again because I had a bad experience with her attitude when DS was younger.