Your postpartum body may be very different than the body you had before getting pregnant. For instance, you probably had abs at one time. While this can be frustrating, it's important not to get caught up in thinking "I used to do x, y, and z pose." You will have to work with the body you have now, not the one you had nine months ago.
If you are breastfeeding, you may be uncomfortable lying on your stomach in poses that squash your chest like knees, chest, and chin. You can always ask the teacher for another pose or go powder your nose.
Mom and Baby
When you have a newborn, you are attached at the hip, or at the breast if you are nursing. But if you have someone who can watch the baby for a few hours, getting away to a yoga class on your own will do you a world of good. It's almost like getting a spa vacation to be able to do something to nurture your own body and soul once or twice a week. If you are lucky enough to be able to do this, be sure to tell the teacher that you just had a baby -- yoga teachers really need to know these types of things in order to offer you the best experience.
If a solo class isn't in the cards for you, don't despair. Mom and baby classes can be wonderful too.