Just to expand a little on what has already been said -
There were a couple threads a month or two back. The SD channel was always part of the sports package but for some reason the HD channel wasn't. Because I knew I never had the SD channel, I never even realized I could have been watching the HD channel until the news release.
But as someone stated, the HD channel was moved to the sports package (where the SD channel has always been). At the beginning of the season there was a free preview of that channel and the NBA League Pass for a few days, so that might have been when you were watching last week.