mat girl,
This may sound weird, but is there any possibility that you are malnourished?
There have been points where I have been malnourished without even realizing it until I went to the doctor. I'd get bloated up a lot. You'd notice a loss of weight / muscle though.
If your diet is bad, it is possible to eat a lot, feel full often, and yet still suffer malnourishment. It's quite common actually in 'rich' countries. People tend to fill up on junk, sugar, and carbs...
Anyhow, a vit/min fix-up can't hurt. Make sure you're getting everything you need.