I think it could be a bit of both.

You probably still have feelings for him especially if you didn't really want the relationship to end. Seeing him with someone else is going to hurt and knowing that he may be doing things with her that he did with you, that could be tough to deal with.
On the other hand if you wanted the relationship to end and you were okay with the break-up it could be just jealousy, maybe you feel annoyed at the fact that he's moved on so quickly and seeing that he could be feeling something for someone else so quick because the two of you broke up after a whole year of dating.
What about you, have you moved on? Are you seeing someone else?