nat question

[Vlad GURDIGA]

Hello,

I could not figureout the answer to a question. Here is the situation:

PC A: Windows XP Pro.
PC B: FreeBSD 6.1, connected to internet, acting as a gateway for PC
A, with NAT (built by hanbook instructions
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/network-natd.html),
open firewall, no restrictions.

For long time I have used the PC A with PC B as gateway and everything
worked just fine, but now PC A can only ping any host (by IP) in
Internet. No other traffic (DNS queries, FTP or HTTP) does not reach
the Internet comming back with TTL exceeded response apparently from
de destination host (I've seen this on PC B with Ethereal).

Question: Is there any way my ISP can 'see' and cut out NATted traffic
from PC A letting only the traffic from PC B pass?! How?!