Hello,
Genetic, environmental(lifestyle liekeating and sedantry lifestyle), psychological, and other factors may all play a role in the development of obesity.
Medical conditions linked with such kind of obesity are hypothyroidism, Cushing's syndrome, depression, and certain neurological problems that can lead to overeating. Also, drugs such as steroids, hormonal birth control and some antidepressants may cause weight gain.
You need to get all these possibilities evaluated.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Talk to your doctor, and get your thyroid checked while you are at it. It might not be your IUD.