Your heart rate of 110 beats per minute and mild chest pain may be related to your mild anemia or your thyroxine dose, which might need a simple adjustment. Additionally, your stomach ache is a common, mild side effect of iron supplements like Bizfer XT. For the best results, take thyroxine on an empty stomach, and space your Shell Cal and Bizfer XT at least four hours apart so they do not interfere with each other. Please rest, stay hydrated, and consult your doctor soon to safely evaluate your chest pain and check your thyroid levels.