Oh, Di, I wanted to cry for you reading over your post. I know how frustrating it can be listening about other's perfect pregnancies. When my SIL told me she cried about not being pregnant anymore I honestly couldn't even understand what she meant! It's hard, too, when you're a first time mom to not compare yourself and/or your baby to others, but try not to. I really doubt that any of the other moms are any more together than you are and those play groups can sometimes bring all those insecurities to a head. You probably made the day for the mother who loaned you an extra outfit. She was undoubtably worrying about something she felt was not quite up to par about her or her baby, and helping you out probably boosted her confidence.
As far as what to say, be vague and smile. I usually say something like, "I don't really enjoy pregnancy, but I adore my babies. Of course, they made it all worth it." If someone presses you say, "Oh, I have a complication of pregnancy. I don't like to go in to it because I thdon't like to be one of those women that just has to share her pregnancy horror story." Most women will get the hint after that.
Good luck! Jazmin is so beautiful. Enjoy her!