Expression is no guide to what you make for your baby. Feeding stimulates hormones and let-down in a way expression just can't. If you suckle more, you'll stimulate supply more than extra expressing, I think. Expressing stimulates, obviously (it's how my son is fed, and at 13 weeks he weighed well over 13 pounds) but not in the same way as your baby suckling. It's pure supply and demand, without the emotional response factored in. If you have any supply worries, and you can comfortably bf, I'd definitely choose to bf more.
Also, are you expressing after the first feed or two when you wake up in the morning, as you usually have the most milk first thing? (Sure tiktok will say if that's wrong!)
I think fluids can matter, though not calories - but only if actually dehydrated. If my mouth is dry and my pee is orange, my supply drops. Drinking extra when not dehydrated, and I just pee more, so that doesn't help me.
Stress and worry makes it harder to get the milk out, so a nice chocolate eclair and tea before starting the session is my therapeutic remedy of choice. And a good DVD can help, too - sometimes I get stressed by a slow flow, then get distracted by the film, and suddenly realise I'm about to overflow the little bottle!
(Madmouse, I hope Monday went really well for you.)