We have two spayed 1 year-old sisters, Iggy and Ziggy.  They were bonded when very young, then separated before being spayed.  After a month of attempted bonding resulted in a fight (despite mostly getting along), we separated them for a few months.

We then had them boarded for two weeks while we were away.  The boarder bonded them, and at no point saw any signs of aggression whatsoever (besides occasional mounting from Iggy, which Ziggy tolerated).  

Now that they are back home for a few days, we have their pen in a different space than before they left, and they are perfectly fine there.  However, when we let them exercise in the living room (where they had previously exercised separately) we see some signs of aggression.  Attempted mounting has turned to circling a few times which we’ve had to break up, and they seem a bit on edge.  Iggy seems to believe the parts of the living room to be her territory, and will chase Ziggy off.  However, as soon as we send them to their pen, they’re completely relaxed again and no longer show any aggression towards one another at all.

Has anyone ever seen anything like this, and does anyone have any ideas?