Hi!
Sorry to hear about your toilet troubles.
Welcome to the board! If you post your question here:
http://www.askmehelpdesk.com/plumbing/
I will bet you that you question will be answered very soon.
What I think is either one of two things, you have a suction sucking the water out of the toilet, or you have a small crack in the bottom of the toilet where you cant see it.