Generalised itching in your case could be due to several factors: poor blood sugar control from diabetes, impaired kidney function (which can cause waste buildup in the blood), or a reaction to your heart medications. Because you have multiple complex conditions, self-medicating is unsafe. Over-the-counter antihistamines might interact poorly with your heart or diabetes drugs. Please consult your doctor immediately. They need to run blood tests to evaluate your kidney function and blood sugar levels, identify the exact cause, and safely prescribe the correct medication or adjust your current treatment plan.