How much caffeine can cause heart palpitations?

No, hon, not an external thing. You have been drinking coke for a year. Coke can definitely cause the heart palpitations and even more so when you have been on a steady diet of drinking it. And not only that but caffeine isn't the only problem. Drinking coke or any soda that contains citric acid, starts to deplete the body of minerals like calcium and magnesium. Loss of those minerals can cause tiredness, weak muscles, shortness of breath and palpitations. I strongly suggest you cut back on the coke on a daily basis. And get some calcium/magnesium supplements to take. Try this for the next week, cut back slowly from 2 glasses of coke to 1/2 glass by reducing the amount you drink each day. Accumulated usage of a chemical like caffeine can act as an addictive drug, so take it easy. See how you feel after the first week and if you feel okay, go for a second week. If you are feeling better then you need to stop and drink coke only occasionally. You know, you can do things for awhile, but the body starts to react to things that cause harm, and the heart racing and shortness of breath are signs of rejection - your body wants you to stop. And it doesn't matter that it was a problem before because it is a problem now. Problems take time to develop.