Getting really depressed about my weight

I lost a lot of weight to be able to get pregnant. Over 100 lbs. It was the best I'd felt and looked in my whole life. During my pregnancy I gained 40 lbs even though I was extremely active. I worked full time on my feet 8 hours a day up until I went into labor and I hiked with my husband on the weekends. I'm 5 months pp, exclusively breastfeeding my daughter and I don't eat super healthy but I don't eat bad either and I only drink water. No coffee. No pop. Juice rarely. When I saw what I weighed at my 6 week check I decided to try to lose weight again. really try. And I did, I watched what I ate and started exercising every day, upped my water intake and I was so excited to weigh myself at my daughter's 4 month check up but when I got on the scale It showed I had gained 15lbs. I broke down and cried right there in the clinic. What the actual fuck. Why am I GAINING WEIGHT? I love my daughter more than life itself and I am so greatful to be able to be her mom, but I'm slowly slipping into depression because I feel so disgusting and disappointed in myself.