Constipation, Pressure In Chest, Palpitation. Prescribed Magnesium Citrate. Frequent Bowel Movement, Severe Cramping. Possible Reason?

I have been experiencing constipation for over a month. While being constipated it felt like I had pressure in my chest, palputations, PVCs, and the PVCs were accompanied by a rapid heat rate. Nothing I tried was working until I visited the ER and they have me this liquid medication called Magnesium Citrate . I took the whole 10oz bottle and swallowed it as directed, and then drank a large cup of water. Several hours later I had a few bowel movements minutes apart, they were largely water and very soupy.. and intense cramping/pain for about a half an hour until the bowel movements were over. So I layed down after that, and when I woke up I felt better for a little while, and then I experienced rapid heartbeat , and had a pain in my left root. The pain turned into what felt like a blood vessel bursting. I can t stand feeling like this but EKGs, Xrays, and tests a cardiologist did reveal no problems, but I get these episodes like the above described and it is really scary. Not sure what is wrong with me or what to do.