Thin hair helpppp

My hair is so thin.. Like super thin. Even have bald spots. I used to have a head full of healthy hair, started losing weight and lost it all. Then when I got pregnant it started getting somewhat thicker and it was so healthy. A year after having my baby its at its worst. What can I take or it? Or use? Vitamins don't seem to help. It seems to look healthier when I have a short cut but I'm tryna let it grow.. Any hairstyles you recommend also?