Why doesVuze (Azureus) constantly changes port numbers?

When i open Vuze, i get an error message that says that there were upnp device problems and that my ports have changed. It does this every time i open Vuze, so every time, i get a new set of port numbers (TCP & UDP). I have gone into my Linksys router to forward the ports, and i have also unchecked the "block anonymous internet request" box that is located in my router, but it doesn't help when Vuze changes the numbers again on it's next startup. When i do start it up, after it tells me about the upnp device problem, i do a "nat test", and both numbers check out fine. So then i have to go back into my router and forward those new port numbers, only for them to change again at the next start up. Why is Vuze constantly changing the numbers? Thanks.