Depo makes you gain weight?

Hello girls! 
So I've been dealing with extreme weight loss and me and my doctor has made try from vitamins to different kinds of shakes like ensure, pedylite, eating more protein etc and she has tested me for different things but we don't seem to find the solution for me to gain weight I lost 40 pounds during my pregnancy and I'm underweight and not gaining as soon as I had my baby 8 months ago my stomach was flat so crazy I know. Well anyways I've never taken any kind of birthcontrol and my doctor has suggested I give it try because it might help me gain weight? I don't know what to think of this since I've never tried it but it would help so much if you girls tell me more about this gaining weight thing and birth control topic, thank you in advance! ❣️
By the way I'm 5'6 and I weight 103 I've always been around 135-140 and I've stayed active. Now I'm so skinny and I'm even taking fluoxetine because being skinny has lead to depression since I've been constantly told "omg you're so skinny are you on a diet? Do you not eat? You looked better thicker..." etc. Makes me sad. 😞
Edit: To those saying I won't have more kids I don't mind because I'm not intrested in any more. First and last. 
And nothing is wrong with my thyroid either.