What Causes Palpitations And A Feeling Of Collapse?

Hi gd morning. I am a female aged 49years. Have removed my uterus 2 years ago due to multiple fibroids inside and outside the uterus. I am not on any hormone replacement theraphy. I get palpitations and feeling of collapse. But upon conducting some blood test everything is normal. Sugar and pressure level is normal. Thyroid normal. What do you think is the cause of the palpitation.