11 yr old sick 2 nights. then ok for 1 week, sickness back accopanied by cold sweats and feeling over hot. stomach bug or something more serious?

Hard to label: Generally any "virus" that has resolved for a week does not return.The reason it left is the natural immune system terminated the virus. Any new symptoms after a week clear is most likely to be a new process, although the systems prior illness may have made it easier to catch. If it is interfering with life/function have it evaluated.