Low Supply in Evening

piper09

I'm concerned that my supply is too low in the evenings at DS's bedtime feed.  He is 4.5 months old, and has been EBF on demand.  I haven't had any supply issues, but for the past week or so, he has been extremely frustrated at his bedtime feeding, fussing and popping off the breast.  

He eats every 2 - 3 hours during the day and during the night.  He had been sleeping much longer stretches at night (up to 10 hours), but that stopped when he was about 3 months old, which I attributed to the 3-month growth spurt, Wonder Week, and then 4-month wakeful, etc... 

I have read that it's possible that even when it seems like your supply is low, it's actually not...but I'm not sure what else the problem would be?  Any ideas?  TIA!