Hello anta,

Hashimoto's disease is common cause of hypothyroidism.  But your symptoms are much more consistant with HYPERthyroidism.  The symptoms of hyperthyroidism are as follows:  weight loss, anxiety, intolerance to heat, fatigue, hair loss, weakness, hyperactivity, irritability,nausea, diarrhea, sweating, and palpitations.

Sometimes hashimoto's thyroiditis can cause symptoms similar to those you are having, hyperthyroid-like, especially if you are early on in the disease and in an acute attack.  When the antibodies at the center of your disease attack your thyroid gland, it can release a significant amount of thyroid hormone which makes you hyperthyroid.

However, if you are on thyroid replacement medications, I wonder if somehow you are taking too much medication?

I doubt you are having a heart attack, or that these are cardiac causes of chest pain.  Back pain accompanied with the rest of your symptoms is much more likely to be a systemic disease. I am even less worried because you have had what sounds like both an echocardiogram and a stress test recently and because you are quite young.

You should absolutely make an appointment with your endocrinologist, or whomever follows your hashimoto's, to discuss these symptoms and how you can better manage your disease. 

To your health,

Neil MD