Yes you should be concerned. Weren't there any posted warnings? you are not supposed to stay in there longer than 5 - 10 minutes at a time . While in a sauna alot of the blood in your body will move to the surface of your skin. This means there is not as mmuch blood circulating to organs including the heart. Also you shold never go after eating. Since it takes blood flow to your digestive tract to digest food, by sitting in a sauna you effectively pull blood away from digestion and it can cause you to pass out, hence why you are not supposed to go alone. It is normal for your heart to speed up , but not that fast. Sauna is good detox though. It removes far more toxins thatn your kidney's can remove.