My pulse rate fluctuates significantly and is irregular. Could this be a health issue?

My pulse was about 50 to 60 beats in the morning, but at lunchtime, it suddenly went up to 80 beats even while just sitting still.
I was sitting still after a meal when I was surprised to feel my pulse rise.
I am worried because my pulse rate seems to fluctuate significantly and is irregular even though I haven't exercised or moved around.
I wonder if this is a natural phenomenon that occurs with aging or a sign that there is a problem with my heart health.