Brain fog is terrible. I don't have it constantly anymore, but I have my days. All I can say is try to stay busy, which is much easier said than done.
As for the heart rate machines at the store, take them with a grain of salt. They often times aren't accurate. Especially if they haven't been recently calibrated. Go see your doctor to get an accurate reading on your heart rate (e.g. when I use the store machines, It always tells me I'm pre-hypertension/hypertension, but everytime I get tested at the GP office, I'm well within the "normal" healthy range.)