Try and stay strong, as if it is a bug you will naturally be feeling low as you are poorly. Give it at least a few more days and see how you feel. Maybe setting the limit of a few days and just focussing on one day at a time will help you get through. This first week really is a killer, I know.
If you have picked up a viral tummy bug, it could be airbourne, so your family may not necessarily catch it, or they may have previously had the same strain, so now have some immunity to this one - lots of reasons they wouldn't have it.
Stay strong xx