It is often difficult to answer questions like this without being able to see all of the clinical details. I cannot speak much about potential issues regarding stopping Trimezatadine as this medication is not approved by the FDA and in not available in the US. I would recommend discussing this with your doctor.
In regards to unstable angina, if this is an intial diagnosis, it generally would require either a cardiac catheterization (coronary angiogram) demonstrating blockages in the coronary arteries causing lack of blood flow to a particular area of the heart muscle or a stress test that indicates an area of the heart is not getting adequate blood flow during stress. There are alternative causes of chest pain that do not involve the heart (heartburn, esophogeal spasm can feel similar) though if you are continuing to have intermittent chest pain that radiates to your shoulder and shortness of breath with exertion, I would definitely discuss this with your doctor in the very near future to see if further testing is required (stress test or catheterization if not done yet). I hope this helps.