For the past few days i've had diarrhoea, nausea, occasional stomach cramps, heartburn and feeling fatigued. do you know what it could be?

Gastroenteritis: This has features of a viral gastroenteritis which should resolve in a few days. Treatment consists of rest, Tylenol,drinking adequate fluids including flat room temperature ginger ale, avoiding harsh roughage, spicy and greasy foods and taking kaopectate or imodium as needed for control of troublesome diarrhea. Prilosec or zantac and avoiding caffeine,alcohol and nicotine should relieve heartburn

Virus: The symptoms you describe sounds to be a viral illness. Food poisoning is also a possibility. These symptoms could last up to 5 days or slightly longer. If the symptoms are lasting more then a week and not improving at all I would recommend seeing a physician. I also would seek medical attention if you start running high fevers or become dehydrated. Good luck