I second the recommendation for an extra snack - one high in protein...I personally try to avoid the 'protein bars' because they tend to have more sugar than I care for.
Make sure you're getting enough water! Rule of thumb is Drink 64oz of water, and to thirst after that. If you don't keep your fluids up, your body will have a really hard time making milk for your babe. Water and teas count towards your daily 64+oz of fluids. Milk, coffee, etc do not.
How long are these runs? If you're missing one or two nursing sessions, then it's more an issue of Supply changing to meet the Demand, or lack of it. Your 3wk old is still needing to nurse about every 90 minutes. That's how long it takes breastmilk to digest...and those feelings of hunger are new and uncomfortable to your baby, so she likes to keep her tummy filled.
You said your supply is low the next day - Do you think that because of how much you're pumping? Or because your babe is nursing more often the next day? IMO, unless you had serious supply issues at the onset of your nursing relationship, there's nothing much to worry about.
There are lots of things you can do to increase milk supply (pills, teas, herbs, etc) -- but my advice to you is to double check your caloric intake and your water intake first.
LLL says to nurse early and often to ensure a well-established milk supply. Your local LLL Leader is there to help. They are volunteers with lots of bf-ing experience.
http://www.lllusa.org/AROK/oklahoma.html
HTH and if anything I said needs clarification, don't hesitate to email me
K., mama to
Catherine, 4.5y
Samuel, 17m