If I were you, I would be happy about that, as it probably means you have excellent form.
I assume you are shooting instinctively, as the problem would be easy enough to correct if you were shooting sights.
It probably means your head is tilted a little differently, so your rear sight, your eye, is not quite in the same position in relation to the arrow as it was when you were shooting spot on. Or it might mean a number of other things, including a slight change in your anchor point, or any changes in equipment you might have made recently. Whatever it means, since you are shooting good groups, you must be doing it consistently, at least for that round of shooting. Which means that if you continued to do it consistently, your groups would gradually move over to the bulls eye as your mind adjusted, similarly to the way your mind adjusts when you switch bows.
The same thing happens to me at times, and all I can say is that it is preferable to suddenly finding that your arrows are skattering all over the target, which also happens to me at times.