This would tend to confirm that it was not your ISP but some attempt by a port scanning bot trying to find computers that are responding to queries (pings etc.), once any port on your computer answers an inbound request, this is likely to lead to further attempts during that connection period.

With a dynamic IP address the next time you connect the process of finding you again would be lessened. But the poor hick that gets the randomly assigned IP address that was port scanned (your previously assigned IP) could be in for a surprise if he isn’t protected by a firewall.