Welcome to the forum, Julie.
Check the archives (the search box at the top of the page) for tons of great threads about sulfites and keeping a food journal to diagnose the sensitivity.
Pretty much all cellulose today is sulfite free. There are some references in the archives.
It only takes a little bit longer to cook oats than the slimy quick stuff. I'd buy the kind that takes 15 minutes if it was available around here. Gives you something to chew on. Not sure why you react. Oats may trigger gluten reaction because it can be cross pollinated with wheat - unless labeled gluten free. There are several levels of gluten sensitivity besides the serious celiac, and they can all be acquired as an adult. Definitely see your allergist about that though!
There are many breakfast cereals labeled gluten free, including oat cereals, but you have to read the ingredients carefully to check on sulfites. Beware especially of molasses and brown sugar! Look up each ingredient until you know the sulfites ones. I pretty much stick to rice crispies if I want a change from old fashion oatmeal.
Atenolol has povidone, which has iodine. Both your drugs have a little starch, which could be of various types, some with sulfite, but there shouldn't be enough to make a difference. Reactions to your drugs are common without sulfites being the cause. See drugs.com or talk to your doctors.