I know it's frustrating, but most likely this is just water weight. When you start dieting, the amount of glycogen stored in your muscles and organs drops pretty dramatically since you're not taking in enough carbs to replenish the glycogen. This depletion of glycogen also causes scale weight to drop in the form of water loss. Over time, those glycogen stores are renewed, which also causes you to put the water weight back on.
This phenomenon is even worse if you also add in new exercise, as your body needs extra water in your muscles to help repair the micro damage that exercise does to your muscles. If you feel sore muscles, you can know for sure that you're storing extra water.
I suspect there's another factor at work here: stress. Elevated stress levels causes you adrenal glands to make and release a hormone known as cortisol into your bloodstream. Among other things, elevated cortisol levels can actually slow down your metabolism, meaning you're burning fewer calories at rest.