I have fluctuations in my heart rate. Is it a life-threatening condition?

Hi doctor,

I am 33 years old female with a weight of 65 kilograms and a height of five feet four inches. I have done a holter test when I was 24 years old. The doctor told me that I have tachycardia and he gave me Diltiazem. I quitted it after taking it for some days. During sleeping, my resting heart rate seems to be around 55 to 65 beats per minute and it was consistent. Now, I notice a drop in my heart rate with 40 beats per minute and it gets spiked up to 150 beats per minute while sitting. I calculate it on my apple watch but it does not monitor my heart rate continuously. But I have a low heart rate of 40 beats per minute once a week and it seems to be high sometimes in a week.

I have shortness of breath with weakness in my arms. When I stand, my vision seems to be black sometimes. After eating, these symptoms become worse. Before few days, I took a small granola bar and my heart rate got raised up to 148 beats per minute. It pounds in the center above my clavicle. I notice my heart rate dropping to 40 beats per minute while eating icecream. I have fluctuations in heart rate while taking cold smoothies. So cold triggers it. But it happens without cold also. Rarely, I have a hard kick on my heart against my chest. I urge to tighten my chest after standing and after getting strange sensations. It lasts for seconds and stops when I do tightening. Sometimes I have chest pain and it lasts for ten seconds. Should I need to be concerned?