WOW- what an overwhelming amount of responses you have!
I, too, was in the same situation as you. I actually had an "on-call" pediatrician in the hospital tell me I could not breastfeed my daughter on the second day of her life because she had lost weight and that I should definitely formula feed her. Luckily, I had done nothing but read about babies while I was pregnant and knew that babies lost weight after they were born. This doesn't mean that he didn't undermine my self-confidence though!
After taking her home (still exclusively breastfeeding her!) I found a pediatrician who was pro-breastfeeding and would let me bring her in anytime I wanted to weigh her, which really helped. I was also pretty obsessive about counting her diapers, which helped my peace of mind on a day-to-day basis!
My 8lb 9oz baby is now 6 years old, she was breastfed for 5 months exclusively, nursed until she was 3.5 yrs old, and eats like a horse now, but guess what? She is still only in the 10% percentile according to the pediatrician's office and weighs in at a whopping 37 lbs- that's right. I know some 3 yr olds who outweigh her! But she is healthy as can be!
The moral of the story: Trust your instincts! If you want to exclusively breastfeed, visit a LLL meeting, call the hospital and ask to speak with a lactation consultant or find one online. One visit is expensive, but it is WORTH IT! Feed your baby on demand- this might mean every 2 hours, or every 10 minutes for awhile!
Also, something that helped me- I had a very sleepy baby who was so comforted by the breast that she would just "sip" and then fall asleep there (but nowhere else! LOL) and the LC told me to gently massage her shoulder to get her to reflexively suck- it worked! I would just very gently rotate or "shrug" her shoulder and she would rouse just enough to begin nursing again.
Don't give up! Don't let one person's comment make you feel like you are starving your baby or not doing the best thing in the world by nursing! I know how fragile we can be in our self-confidence when we have just given birth, and you need to find encouragement right now, not DIScouragement!
Best of wishes to you and CONGRATULATIONS on your new baby!!!
-I'm also an aspiring lactation consultant if you couldn't tell! :)
(you can feel free to email me anytime at ____@____.com)