Dr Zeinab  Al-`Alawani, Instructor of Fiqh and Islamic Studies,Graduate School of Islamic and Social Sciences, states the following: As far as I know, each case is different when it comes to fasting while breastfeeding. While some women are capable of breastfeeding and fasting, others are not. As you are not comfortable about the advice of the doctor you consulted, you are free to take a second opinion from a Muslim doctor that is you think would be more capable to understand your situation.
as far as the shari`ah rule on your case, if you are capable of breastfeeding while fasting, you may fast. But if the milk will be affected or you will be harmed, then you may not fast.