I went through radioactive iodine treatment for thyroid cancer a couple years ago. You are the most "contagious" for the first few days, but each day get less and less radioactive. I stayed in seclusion in the hospital for the first 3 days until I was considered "safe" to be around by a certain distance. They determined this by waving some sort of wand in front of me to detect my levels. When I was discharged, I stayed with my parents in a separate room, just wiping things down I touched, and just had to stay a few feet away from them. I stayed with them because my son was 8 or 9 at the time and didn't want to do anything to expose him, plus my DH has a history of cancer with radiation/chemo, and didn't want to add any more substances to the mix. After a week I went home with no restrictions.
By 3 weeks, both you and she will be fine around each other. I got the impression it was just the first few days that were the worst and even at that to have any sort of negative impact on you, you'd have to be in super close contact for a very long time. You will be fine at 3 weeks.