No one gains weight like me! :/ Help?

I don't know whats wrong with me.. I'm barely 18, had a baby in December and gain weight faster than I blink. I'm 5' 4", have a medium build, and am normally a size 5 at 116lbs. That was 18 months ago. Today I weighed 188 pounds. I put on 70 pounds with my son, lost 20 when I had him and gained more than that back in the last 6 months. I count calories, barely eat carbs and red meat, exercise as much as I can but every time I loose a pound I gain it right back. Any suggestions?