As a lady with an ample bosom (36 D) that tends to be more sensitive to pain than many who also had low milk supply issues nursing her first child...
I was advised that it is VERY IMPORTANT to wear a well-fitting nursing bra, especially during the early stages of nursing. Later, if your supply is good and you aren't prone to clogged ducts, then you can safely try pulling aside the cups on other types of bras. (Advice from an IBCLC.)
I pray that you have no breastfeeding problems! They stink! But I do like to share this info with new moms, just in case...

I love this particular web store for nursing bras.I've never seen as much selection elsewhere, and they ship for $1 and encourage you to order lots and return most to get a good fit:
www.birthandbaby.com
Last time, I maxed out at a 38 G. Crazy big! I'm about a 36 E right now... Nursing bras allow much more room for cup size expansion/contraction than typical fashion bras, so you don't have to buy quite as many as it sounds like. Whew!
Blue Canoe (brand) "Jane's Bra" (plus size cup) and "Yoga Bra" styles work for me as backup bras (for those super-sensitive days) and also as sleep bras. Since I want to encourage you to shop really, really carefully for an excellent fit in a nursing bra, I offer these as ideas to keep you from being forced to go painfully braless in the meantime. (It would be painful for me. Some women seem to do fine without a bra.

Baffles me.

)
Looks like Birth & Baby carries some Blue Canoe products, too, though I got mine elsewhere. Also, I just found a new nursing bra (Melinda G brand) that I hope will be my "comfy days" bra for this time, because I had latex allergy reactions to my Bravado double+, and they weren't really supportive enough for me anyway. If you like that idea but want a touch more support, look at the Melinda G:
http://www.birthandbabyorders.com/index.php?c=53
--willo