Do you have chest heaviness and discomfort?
Any pain in left arm or upper abdominal pain or dizziness?
Is it possible for you to take pulse and blood pressure?
A diagnosis of heat exhaustion is dependent on exposure time to heat, apart from your symptoms. If you feel that you had lost, lot of water and electrolytes during shopping, and was unable to compensate the losses, the heat exhaustion would be the diagnosis. It does not look like acid reflux with your symptoms which is not very likely to cause difficulty breathing, weakness and fast pulse rate.
Having said that; with your past history of cardiac ailment, it is necessary that you get an EKG and heart enzymes (which are blood work). Your symptoms need categorical ruling out of any heart related problem, despite no chest pain or discomfort. Vomiting, weakness and difficulty breathing are sufficient enough symptoms for the cardiac evaluation. So going to emergency room, is advised, even if you feel better, to pin point the cause. Thus necessary precautions can be taken in future and treatment can be done if there is any present problem.
Please feel free for your follow up questions.