You know, those weight scales are for FORMULA fed infants. Formula fed infants are usually a little heavier because the body can't break down and use everything in it like it can with breastmilk. Unfortunately, it seems like the medical field is very ill-informed on breastfeeding issues.
I would recommend checking out this site: http://www.kellymom.com/babyconcerns/growth/index.html
I have always heard www.kellymom.com to be a very reliable and trustworthy site. I have a feeling your child's weight is just fine (if it is just a little below what a formula fed child would weigh).
Is your child eating solids yet? If she is that will probably help to put a little weight on her too. My daughter was in the 15%ile for weight for a LONG time. She didn't take to solids until probably about a year but after that her weight went up to about 30%ile.
I wish you the best of luck! And trust your instincts and do whatever you feel is best!!!!
Edited to add:
Many women simply can't get much milk pumping. It doesn't necessarily mean you don't have much milk in you but that your body doesn't respond to something artificial trying to get it out. I never could pump much (no more than a couple ounces) and I had enough milk for my child. This really isn't all that uncommon. And I DID use a hospital grade pump at one point because I needed to pump in the early months a couple times a day so I could put her reflux medicine in it. I never had much luck.
The amazing thing about women's bodies is that their bodies produce whatever amount of milk your child is needing. because she is getting formula right now she isn't nursing as much which is why your supply is going down. But the more she nurses your supply will fill whatever her demands are. Frequent nursing is the BEST way to increase milk production. I personally, and again everyone should do what they feel most comfortable with, would be VERY leery about using Reglan or any other prescription medication. There are more natural methods if you are desperate (kellymom in my opinion would be a good resource here as well). I have also heard eating oatmeal helps with supply.
I also go to www.justmommies.com and they have a breastfeeding board where there are a ton of VERY informed women who would be more than willing to help out in any way possible. It is a pretty well traveled board so you should get pretty quick responses.
And lastly I just wanted to say that the benefits of breastfeeding continue for as long as you breastfeed. I know alot of people would stop at this point but since you said you had planned on extended breastfeeding I would certainly encourage continuing because you and your daughter are both still getting so much out of it (it is amazing all the benefits of extended breastfeeding - it is truly a shame that so many pediatricians aren't up on all the nutrional and medical benefits).
Hang in there! You are doing great!