If you have a tapeworm, this is a parasitic worm which has attached itself to your lower intestine. It will nourish itself from your food, and you're likely to find it hard to keep weight on, and may suffer from lethargy.
Tapeworms are generally caught from infected food, which has been handled by someone who has transferred the tapeworm eggs because of poor personal hygeine. Basically, the eggs are ejected from your system with your faeces. If you get them on your hand and don't wash properly, you could pass them on.
It is not an eating disorder; it's not psychological. If there is a worm in you, it can be got out and you will then be able to keep on weight.