Port randomly closing?

[Narmo23]

Been using the default port for a while. However, as of today, it mysteriously wasn't open -- had the indicator telling me.

I'm curious as to how it closed? I know a thing or two about routers (i.e. opening a port for WC3 so I can host games -- that type of stuff), but I've never encountered this problem before.

Two culprits that I'm suspecting:

Malware - I highly doubt it's this one as my scanner, NIS 2008, has yet to pick up any malware whatsoever. I've been using NIS since '06, and I know that it's pretty damn decent. Can care less if people call it bloatware, as my computer doesn't get fazed by it.

Service provider - Comcast is my provider, so maybe they're behind it?

Edit: One thing I forgot to mention was that I'm using a WRT54G. And well, I know about the patches and what not, but let me say this: this is my first time seeing my port being closed without me doing it. And no, my speed is definitely good enough; I've seen some exceed a MB, which scared me a bit.

Edit #2: Forgot to mention that I randomized the port after this happened; everything is going normal. However, I still want to know what caused the port to close.

[Ultima]

If you forward your port on a WRT54G, you better make sure you forward it to a static IP outside of the router's DHCP range. Linksys routers are known to simply ignore port forwarding rules that target dynamic IPs.