Probably the definitive guide for meds and breastfeeding is this book (you can search on it too): http://www.amazon.com/Medications-Mothers-Milk/dp/0977226...
Keep in mind too, just because they're an MD doesn't mean they really know much about breastfeeding. That's where Dr Hale's book comes in handy. I also think LLL is a great resource.
Regarding which Dr to see... Whichever one makes best sense. In theory, they should all be knowledgeable regarding which meds are ok for breastfeeding moms. If you have a sinus infection, you'd see your GP. For something post delivery, your OB.
I personally found my OB to be the best resource for breastfeeding issues simply because she had done it 3 times and was very pro breastfeeding. My GP was also very supportive of my extended breastfeeding. If I had listened to my son's ped, who knows if I would have breast fed for as long as I did. A lot of Dr's say they're pro breastfeeding, but when it comes down to it, really aren't. Sad but true I've found.