before you decide ANYTHING, go to www.llli.org. contact a local or nearby la leche league leader. she can help you determine if your supply is decreasing, what to do to increase it, and then she can help you to figure out how and what to suppliment. its likely that you dont have to, you just might have to make some adjustments. to me, it just sounds like you are more worried that you arent making enough milk that whether or not that is a fact.
some important questions to ask: are you ever feeding your daughter a bottle when you are present and able to nurse her? the problem with pumping is that it doesnt stimulate your breast the same way, and your milk supply is determined by the nursing from your baby, not from a pump. sure, it helps, and its a very good thing, it just doesnt stimulate you the same. make sure that you are nursing on demand, anytime you are with your baby, NOT giving her any bottles.
also, if this is during a growth spurt of any kind, your baby will not seem like shes getting enough. this is normal, and part of the process of increasing your milk. like i said, let her nurse on demand, and no bottles when you are with her, this will help tremendously to increase your milk when shes growing. also nursing at night is important, that is when you are most relaxed, and babies do typically still need to nurse at night, so dont be afraid to do it. cosleeping, or bedsharing is safer than most people think, and its really beneficial to your breastfeeding relationship. i wouldnt wake her to nurse, but dont be afraid to nurse her if she wakes up. shes old enough that you shouldnt have too many worries about suffocation, but also be aware of blankets and pillows. a lot of times, you can literally get her latched on and all while you are mostly still asleep. instinctivly, you will know where she is in bed, unless you are under an outside influence of a drug or alcohol, or whatever. so dont worry about it if you feel like giving it a try. its almost a work-free way to increase your supply. :) it takes some getting used to at first, but i found it the best thing i ever did.
anyway, if you do NOTHING else, contact a la leche league leader or attend a meeting. i had to drive an hour to the nearest one to me, but it was WORTH IT. there are a TON of resources available to moms from the lll, and also the priceless information from other moms who have BEEN there. :) you wont regret that. :)
good luck.