Dropping night feed...supply issues?

[Starburst2]

Hi my lo is 4 months and tends to cluster feed from about 5-7.30 we have introduced a bottle of formula from about 6.30 so my hubbie can feed him and gives me a bit of time to walk the dog or go to the gym. He goes to bed at 7.30 and either finishes the bottle or has a small bf to go to sleep but would hardly take anything. I was pumping in the evening and only getting about 1-2oz and getting tired of doing it so stopped.Lo sometimes sttn or has a small feed at 5 and back to sleep but would only take the one breast and then I express the other in the morning. I usually get about 5-6oz when I pump but since I stopped expressing at night I am only getting about 3oz in the morning. Do you think my supply is dropping?