Another possibility is BPPV, where ear crystals get dislodged.
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002393/
You can test for this and actually self-treat at home if you have someone to help you. Here's a video showing how to do the test and treatment:
http://www.youtube.com/watch?v=59EIKztATiw