Re: How low of a weight would affect my BM?
Follow your body. There is no set number that applies to everyone. Some women have problems if they aren't carrying at least 10 extra pounds. Others lose a ridiculous amount of weight breastfeeding and are WAY underweight but manage just fine.
Watch your baby's weight gain, wet/poopy diapers and satisfaction. Unless you notice a change, then just be happy that you got there without having to diet!