It would seem that your symptoms, though it would have initially started with hives due an allergic reaction, now seem to have extended to your heart. This is explained by the tightness in your chest and neck. Given your age and history of valvular disease, this is quite likely.
I would recommend that you get an ECG as soon as possible, and consult a cardiologist as soon as possible. Tiredness, difficulty in exercising and chest tightness in a person of your age are classical signs of ischemic heart disease.
It could also be that Benadryl might be causing some of your symptoms, as it is known to cause severe drowsiness and sleepiness.
Please do not hesitate to contact us if you have any further doubts
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