My breastfed 8 week old DS is generally pretty content, until the end of the day (usually from 4-9pm) where he is quite fussy and unsettled (I'm assuming is something colicky related) and when I try to feed will only take the left breast. If i try on the right side he wont even attempt a suck, just cries instantly..? I wouldn't mind so much except he's a little underweight, and although he's gaining its dropping through the centiles slightly.
There have been times in the past where the right side has made him choke & gag from strong letdown. Most feeds I offer right first as the letdown is much slower on left side. If anything my boobs feel much emptier from 4pm and letdown is nowhere near as strong as earlier in the day.
Eventually somewhere between 9-10pm he will accept both again, fall asleep then won't feed again til 5am, where he'll have both happily again.
Never came across this problem myself, but as an experienced breast feeder I have a few thoughts. Firstly, is it possible that he is uncomfortable feeding from that side because the position makes his colicky pain worse? My daughter would never lie on her right side when she was gassy as it made her feel worse. To test this, you could try a different position which kept pressure off his tummy. My second thought is that perhaps when he's feeling colicky, the quicker, stronger letdown overwhelms him. In either case, it doesn't really matter that he doesn't want the right breast - if he feeds consistently from the left at that time of day, your supply will accommodate this. A friend of mine with twins assigned them each a different breast and her milk was fine for each twin, even when they fed different amounts at different times. Hope this helps.