I was exercising and when I finished I drank ice cold and my heart started racing really fast!?

May be AF. Although likely normal, rapid drinking of ice cold drinks can rarely cause atrial fibrillation, a rapid irregular heart rhythm. I have treated one young person who had this after a slush drink.
May be ok. A fast heart rate during heavy exercise is ok. Overexertion can lead to problems if the heart can't beat well enough to keep up with a body's needs, so a person should see the doctor if there is chest pain, wheezing, unusual shortness of breath, sustained fast heart rate after exercise, fast heart rate when not exercising, blackouts or grayouts, etc... Ice cold drinks shouldn't change heart rate.