What Causes Rapid Heart Rate With Minimal Exertion?

I am a 35y F. I have chronic fatigue syndrome, fibromyalgia, and early stages of Huntington s Disease. I have mobilty problems so I am not very mobile & cannot excersize. I am mobile in shorts spells. For some time I have a racing pounding heart with near or actual fainting, (also during I feel overheated, sweating, nausea) from minimal exertion such as having a shower, getting dressed, walking or standing for more time than I can handle, or from sexual intercourse even if I lie completely still during! Yes it got to that stage I cannot have intimate relations with my husband even for a few minutes. I always feel seriously ill fast with heart racing, too hot, feel so faint, shaky and weak. I need to lay flat on a cool floor, stripped, fanned & cold water on me to try stop me from passing out, i lay completely still for ten mins & if I sit up too soon I near faint again. There have been times I do actually faint. Should I be worried about my heart or is this signs I am just unfit from lack of mobility? It worries me and is holding me back in daily quality of life.