I felt like you when I was hyper - fast beating heart, short of breath, and always feeling drained plus sleeping problems; it wasn't pleasant. When I was subclinical I was okay but the symptoms progressed.
I did RAI for a toxic nodule, and researched that it was pretty uncommon to get the eye disease, but can happen. They may put you on beta blockers for your fast heartbeat? I put myself in a separate room in the house with my laptop, tv, books, food for breakfast, lunch and snacks and my family left dinner up the hallway in the evenings. I did drink extra water to flush the RAI through my system quickly (and chewed gum to keep the salivary glands working, although I did read that blocked glands are unlikely to happen to those of us taking lower doses for hyper rather than the higher doses for ablation after TT). The scariest part was being scared!!
Probably about 3 weeks later I panicked a bit when my heart raced (I wasn't on beta-blockers), but by the time I got to the doctors I was calm again, so could have been a panic attack. They seem to expect a lot of people to go hypo within the first 6 months, but it took me a year, and, after the first 3 weeks or so after RAI, I felt better and better (until I started going hypo and have had my moments, but still working, and of late feeling great). To date, I don't regret it. I had 15 mci, after deciding to do the RAI instead of surgery.
First remember EVERY one reacts differently to certain treatments.
I had it done for hyper/graves almost18yr ago. I will say I did end up with thyroid eye disease & issues with my salivary gland years down the road. The treatment did not help me so I ended up having a TT about a month later.
I don't recall feeling weird or anything from taking it.Then again it was almost 18yrs ago.