I haven't heard of anyone experiencing that after RAI, especially since you've been on your meds for 13 days but I could be wrong. I think I had every single side effect of the RAI, at least that's what my doc tells me! I had the nausea, the indigestion, the dry eyes, the swollen face, and best of all, my teeth still hurt to this day. I did get head rushes upon standing for awhile but nothing like what you are experiencing. Maybe it's just your body getting adjusted to the medicine. I'd call and mention it to the doctor, it doesn't sound like something to mess with. Good luck.
It is returning back on your SYnthroid most likely along with RAI. RAi will still be working and now that you are on the med again - it is trying to balance you out. 13 days you should be feeling like an adjustment is happening. dizzy spells can be common yet make sure your doctor is aware of it. There are meds availbale to help if it is bothersome.