Merredith,

I would follow the good advice about seeing your oncologist; however, to keep you from panic while waiting, this could be from a number of different things: sinus problems, which are also caused by herceptin; a virus. There's been one going around that occasionally causes dizziness, an inner ear problem, etc. Under normal circumstances, if you didn't have cancer, you would probably think of all these yourself. So definitely see your oncologist, and insist on an MRI, but don't panic. I found Ronda's comment interesting also, that she had the same experience while on herceptin but not when she came off. Good luck.