I guess now that she believes it's serious it's time for you to sit down and think about whether or not you think the 5-month relationship is serious to you. Since she thinks it's serious, then, it's cheating on your part. If you don't think it's serious, I think it's best for you to tell her that so she can move on without too much hurt and anguish. That's the best thing if you don't think it's serious. Just cut it nice and clean even though she might be kind of upset now. If you waited say a year or two down the road, then, the situation might get worse emotionally if you shouldn't last in the long run.