Hello, Thanks for writing to Health Care Magic, I am Dr Asad Riaz, I have closely read your question and I understand your concerns, I will hereby guide you regarding your health related problem. IBS is a condition in which patient have chnging bowel habbit like sometym have diarreha n some time have constipation...fatigue n muscle cramps u having is mostly due to electrolytes problem as u having diarhea whch may lead to loss of electrolytes n result having nsuch features,,if u wre my patient i would like to do electrolytes test n tx it accrdngly..for severe cramps u take OTC analgesic like acetaminophn..family history of heart disease make patient prne to heart disease but it is nt necsasrily that u having or will have that disease in future,thee symptoms are very much sugestive of electrolytes prob n u donot worry about the heart ... I hope this answered your question, if you have more feel free to ask. Regards. Dr.Asad Riaz. General and Family Physician.