I noticed that my right breast seemed to slow towards the end of feeding DS1. It was further down the track than you are experiencing though but it never proved to be in issue for us. I'd suspect that it's your body adujusting to your LO sleeping longer

Once i realised i fed from that side first each feed and we were fine. I'm pretty sure once this happed though the amount he was getting wasn't so much an issue as he was well and turely on solids.

I am no expert and there may be some better advice out there for you but i'd continue to feed first from the lesser side to try and encourage the production.

And for what it's worth feeding DS2 i have never had this issue But he's still having 4 a day at almost 14 months old