It definitely sounds like you have parasites. You need to see a doctor - there is no way to cure an internal infestation without medication, and without knowing what kind of parasite you may have.
Yes, it sounds like pin worms they are quite common and can be in food that you've eaten or from your cats but they enter through the mouth and live in and around your anus. At your local drug store you can buy a pack of pinworm medication, it is not required to have a prescription. I had pin worms about a month ago.