my mini had a 28 gig swap file when I woke up.

I noticed after I ran parallels I had a few hundred meg swap file. not sure why I don't remember ever having a swap file. I have a early 2009 mini with 4 gigs of ram and a 500 gig drive. even running parallels I have never ran out of memory. I just had firefox mail itunes and a few other things I always have. I left hand break ripping a dvd. I get up and my magic mouse is disconnected and things are crawling. get a hard drive space warning says no free space for apps to write to check and I have a 26 gig swap file. no clue I close everythiong down and reboot. seems fine but I notice with 2 gigs free I have a 19 megswap file while ripping with hand brake. any idea's?