Completely outrageous! Don't walk - run to another GP. I would go so far as suggesting that you make an official complaint to the College of GPs. Longterm breastfeeding is the normal state for human mothers and babies. You don't need to do anything special while you are breastfeeding except follow a normal diet. In fact, the body uses nutrients more efficiently, including calcium, and after breastfeeding there is a "rebound effect" so that the net effect is that breastfeeding mothers have stronger bones and formula feeding mothers are at higher risk of osteoperosis. The course of action suggested for weaning is child abuse.
For information on ring Rodney Whyte at the Monash Medical Centre Drug information line on (03) 9594 2361. I feel sure he would advise you you can safely take the medication whilst breastfeeding.
Warm Regards
Barb