Could Waking Up To Strong Heart Pulsations Be Due To Nightmares Or Sleeping Positions?

hello, in the past 2 weeks I have been woken up 4 times by a feeling of my heart pulsating at incredibly strong vibrations. I do not deal with anxiety, and only minor occasional stresses. My heart rate remains low: 50-70 but the volume of the pulsations rises to the point of waking me or my partner up. My heart rate never exceeds 90. From my limited knowledge it could be as benign as nightmares or a sleeping position, but also as problematic as a bounding pulse or aneurysm. Thank you for your help.