I would lean to the other guy route as you have said she had always come back to you which shows she doesn't like being alone, now that she has left for good it would show that she has someone else.
Also even if it is just a fancy at the moment she will probably have someone else on the cards.
KaHn
On a side note and a generalisation I have found that as you have said most women will re-evaluate their relationship but will normally only end it if it is really bad or if their is something better.
Hence the other person comments, but that just comes from what I have seen girls/women do in the past with friends.
KaHn
IMO Kahn is absolutely spot on with the above.