At the end of the day the actual reason for the follow-up call can possibly run the gamut from your most cynical thoughts to your most flattering.  Of course only the lady knows for sure.  However in an attempt to strike the balance between remembering that this hobby involves services offered and rendered and the biz associated with that dynamic vs. the fact that that no matter what two people were intimate with each other and therefore emotions and feelings do come into play, I'd suggest that you view this as a classy nice touch.  It does not happen all the time and I'm sure some gents would be somewhat uneasy about it.  However provided that the call was not intrusive or ill-timed (i.e. violating discretion), it really is complimentary and at a minimum signifies that she would like to entertain you once again if you felt likewise.  Try not to get carried away to either side of the flattery/paranoia spectrum.  It was a classy personal touch and that's always nice to receive.