Could be due to any number of reasons, really -- only you can figure it out.

For instance, it could bring to the fore the rather painful and usually hidden issue of the therapist having a separate life / important relationships / interests etc.

If it's coping with just the time between sessions -- without the knowledge of her being out-of-town coming into play -- there is a way by which clients can get used to it and still be able to live with the lack of contact.

One way of parsing it out might be to see how you'd feel if she were to tell you that she was doing some X, Y, Z time-consuming / fun / important activity over a weekend when she is still in town.

Would you be more okay with that?

If you would be more okay with that compared to her being out of town, then perhaps it is the distance itself -- and the anxiety it likely provokes in terms of her unavailability -- that could be behind it?

Or, some combination of all of the above?