Hi there!
I am currently still breastfeeding my (just turned) 2 year old. I also had a problem with weight loss after my daughter was born. I basically just tried to constantly eat, even if I wasn't hungry, and added one or two supplemental shakes a day. This kept me from continuing to lose, however, I didn't start gaining weight back until she started nursing much less frequently.
As far as the shakes went, I used a chocolate protein powder and mixed it with ice cream and vanilla almond milk. They were so yummy that I actually looked forward to them!
I miss them now that I've gained enough weight I don't need the extra!
I'm sorry I can't speak to the constipation issue. My little one never had issues, always had loose, regular breastfed baby BMs. She was, however, super gassy and seemed to have tummy problems and wanted to nurse 24/7. The doc thought maybe it was a cow's milk protein intolerance, so we cut out dairy and she did WAY better! Of course cutting dairy out of my diet also contributed to my weight loss problem. Ugh. But i did find substitutes, such as almond milk, rice milk, vegan protein powder etc. It took a lot of work, but I'm glad I've stuck it out.
Good luck to you! Take good care of yourself and baby!
Autumn 33 w/CF