Unfortunately I cannot help you out immensely with this one. I cannot see it, nor do I have an immense ball python specific background to rely on.
In general:
Pink belly colouration, be it transient or not, can be normal, the result of a first degree burn, a sign of sepsis or of course, the ellusive "none of the above". As you can see that combined with the normal behavior you probably don't have much to worry about, however some causes of pink belly colouration can be serious. Discuss this on the ball python forum, but just as importantly, see if you can get someone with a good amount of experience to come over and take a look for themselves. There is nothing like a hands-on exam. If all turns out well, then you at least got to show off your collection 
Ian