As you said, it might just be a stage. Of course, if you are her primary caregiver, she's probably going to gravitate more toward you. Is Dad anxious, scared, mad that she acts this way? Do you make a big deal out of it? That could certainly make it worse...the best thing to do might just be to carry on normally as you would, and don't push her about being with Dad. Eventually she'll come around, especially if Dad is particularly lighthearted about the situation. Is there something special that Dad could do with her, like arts and crafts, PlayDoh, something that is off-limits to you and just for Dad only? He could be the "PlayDoh Man" or something.
Hope it gets better! Kids are funny aren't they. But the minute you have it all figured out, something changes!