E.W.
I was in your position, except that I did not breastfeed. My daughter was born 8 lbs 10 oz and did not hit 20 pounds until she was 2.5 years old! She is now 3.5 and she weights 23 lbs. We have been seeing a pediatric endocrinologist for the past 18 months and so far the endocrinologist seems to thing everything is fine. According to the endoc. their is no link between birth weight and the weight that a child "should be". It also does not necessarily matter if a child is small as long as she is healthy. The only reason that we see the endoc. is not because she is small but because she cannot get on a path on the growth chart and stay on it. She would go from 15% in weight to 8% and then kept going down from there.
When we went to the endocrinologist the first time she tested her thyroid, and also tested her for celiac disease. My daughter was negative for both and when she was 2 years old they did a bone density test to see what her bone age was (it was just an X-ray). Since then we have been going in every 3-4 months to get measurements and monitor her growth. The only thing that she tells us to do is increase her calories by giving her high calorie snacks and meals.
I guess through all of my rambling I am saying that if you are worried about then I would see a Pediatric Endocronologist, we see Dr. Sheer. We were really worried about our daughter, Dr. Sheer told us the things it could be and then we just tested her for it. Everything has turned out fine and even though she is small she is VERY bright, just in a littler package =)
Good luck, and I am sure everything will turn out fine.