Have you considered other ideas besides reflux? Spitting up isn't all that uncommon in babies this age...and breastfed babies don't usually gain the same in early weeks as formula fed babies. Babies with reflux usually have other clinical signs (maybe you just didn't mention them here) Unless your baby is listless or exhibiting other signs of dehydration, I think I would ask a few more questions before going right to the medication.
Don't get me wrong, it is doctors that I feel should do a little more problem solving. If you don't absolutely love your pediatrician and feel as though he/she really tried to solve the problem, I believe you should get a second opinion. A doctor that isn't looking for other resolutions at this age is unlikely to support your breastfeeding...especially if your child doesn't gain weight according to their charts. I hope I am wrong, but it is something for you to consider.
One idea is that she could be experiencing a sensitivity to dairy. I had to remove all liquid milk from my diet--still did okay with cheese or yogurt in moderation--until my breastfed daughter was about 5 months old. It was painful for me since I guzzle milk like a baby calf, but it sure made a difference for her. It takes about two weeks for the milk protein to clear your system--I did a complete blitz on all dairy for two weeks and then added back in the cheese and yogurt.
A little bit of rice cereal is commonly suggested for reflux, but I truly believe she is too young. Start with removing culprits from your diet before adding medications and food to hers!
Good luck. I know how rough these early weeks can be.