I have a discomfort in my lower left side of abdomen and can feel a pulse when I am laid down. I feel tense, heart races inbetween.?

Should see your MD. While the symptoms you provide are not specific enough to give you a diagnosis, there are only a few things in the left lower abdomen - sigmoid colon, left ovary, iliac artery and vein, left kidney/ureter and muscle. The discomfort could be due to any of them - the pulse is likely transmitted from the aorta or iliac artery. You may need an ultrasound first to see if there is something of concern.
See MD. U need further evaluation by ultrasound and c/t for the abdominal symptoms. Need hm for the cardiac arrhythmia. See your md now!