Could my weight gain be from my birth control or retaining water?

I am a 27yr old female with 2 children ages 4 & 1. I am very active with my children and eat decently healthy. I have always used the same birth control Ortho Tri Cyclen for the past 11yrs. In the past 8 months, I have gained almost 40 pounds. I am usually at 135-140 and that is a normal healthy weight for me. I now weigh almost 180 and feel miserable. I am ALWAYS tired and fatigued but still make the effort to do everything with my children. A few friends think I am either retaining water for some reason or maybe its the birth control. I was just at the Dr in Feb. and weighed 10lbs lighter than I do now. He did bloodwork to check my thyroid and a few other things and everything was normal.