was your pediatrician concerned about her weight? she may just be petite. so unless there are health concerns, i wouldn't worry. keep in mind that the growth charts are based on formula fed american (usually from caucasian background) babies. since you are primarily breastfeeding, she does not fall into that category.
about your milk supply. every time you give your baby a bottle, you're telling you body that you need to make that much less breastmilk. so if you're giving her 2 ounces, you're telling your body to make 2 ounces less.
sometimes latch can be an issue. you probably think that since you've made it this far and she has gained weight that her latch has been good. but a baby can still gain weight with a poor latch if there is enough milk. see if you can get in touch with a specialist that can check for this. if it is a concern, you'll be amazed at how much more efficiently she'll be able to feed with a proper latch and how much more milk you'll make.
it's not recommended by the academy of pediatrics to give anything other than breastmilk or formula to babies under 6 months. remember, filling her tummy up with anything other than breastmilk is telling your body that you don't need to make more. and you want to make sure that she's developmentally ready for cereal or other foods. your pediatrician will help you determine that.
there are many ways to increase your milk supply. here are a few websites that might be helpful.
http://workandpump.com/supply.htm
http://www.kellymom.com/bf/supply/low-supply.html
http://www.kellymom.com/bf/supply/index.html
http://www.askdrsears.com/html/2/T022800.asp
good luck with everything.