Try avoiding the oats for a few days and see how you feel.
It's also possible that you've developed an intolerance to foods you eat a lot. You could try eliminating some foods one at a time and see if you can find the culprit (if eliminating oats doesn't help).
There are many heavy metals, and silver fillings are just one source of them. Mercury and lead are likely the most common metals, but there are others too.
I'm sorry to say that youth is no protection against heavy metals. Older people may be more likely to suffer if they are exposed to the metals for a longer period of time. But metals can affect anybody; especially if your body is poor at chelating itself.
Chlorella by itself is not an effective chelator. You would need something else to pull the metals from the places where they get stuck in your body. Opinion varies as to what is the best way to do it. DMSA, ALA, DMPS, Cilantro and EDTA are all chelators. They all have good and bad things about them and supporters and detractors. Many of them are effective only against one of the metals. It would help to have some idea which metal may be causing your trouble (of course this assumes heavy metals are the root of your health trouble).
I recommend research into heavy metal toxicity to see if it makes sense, given your symptoms. It sounds like a real possibility given what you've described.