The episode guide at HBO says it was a stroke.
http://www.hbo.com/sopranos/episode/index.shtml
I had some of the same confusion. My take-away is that was a reinforcement of the fact that Tony is business first (i.e. he was glad the debt was settled), personal relationships a distant second (notwithstanding his generally superficial concern for others). In other words, even though we the viewers may find this guy charming or amusing, he is still a cold-blooded pr*ck.