Milk supply and schedule

Hi,
I’m looking for some advice about pumping and my milk supply.
I have a 13-week-old baby. Until a few days ago, I was pumping about five times a day: around 7 a.m., 11 a.m., 3 p.m., and 7 p.m. (the milk from one pumping session would usually cover two feeds), plus a short 10-minute pumping session around 9 p.m. just to relieve the fullness. I also breastfeed twice during the night, usually around midnight and 3 a.m., offering the opposite breast at each feed.
Over the last few days, I’ve noticed that my first pump of the morning yields much less milk than it used to. I used to get around 220–240 ml, but now I only get about 140–160 ml. Even so, I still seem to be producing enough milk for my baby (or at least I think I am). Because of this, I’ve started pumping every three hours instead (7 a.m., 10 a.m., 1 p.m., 5 p.m., 7 p.m., and 9 p.m., plus the night feeds), and I now get around 120–130 ml per pumping session.
My baby usually takes about 100 ml from a bottle every two hours, although sometimes a little more or a little less.
I’m not sure what changed or why I’m no longer able to pump as much milk as before. Does this mean my milk supply is decreasing? Is it likely to continue decreasing? Is there any chance I could go back to my previous pumping schedule and still produce the same amount of milk?
I exclusively breastfed my first child, so pumping is new to me, and I’m wondering if I might be doing something wrong.
Thank you in advance for any advice!