DS has been fussy on the breast the last couple days, not feeding as much and when he does it's not for long or he pulls off all the time. At first I thought it was his teething but this morning AF returned for the first time

For the past 3 days he is averaging 3/4 feeds, but not very long
I'm wondering... is this the reason he is being so fussy? How long does it last? Should I supplement him with formula? (He isn't unsettled and doesn't seem hungry but worried he isn't getting enough milk) or offer more solids? (He has cereal in the mornings)

I have been expressing when he doesn't want to feed to keep my supply up but will he drink that milk later on?

TIA xx