Anxiety attacks caused by weight loss?

I am severely overweight (around 100 pds) and I struggle to lose even a pound and all I have to do is eat one bad meal and the weight is right back. I recently lost about 50 pounds and began having anxiety/panic attacks.

Did the weight loss cause a chemical imbalance? Is the same chemical imbalance causing excessive weight gain (+40 pounds in one year) and making it harder than ever to lose weight?

thanks