Hi Mary,
When I was pregnant I felt exactly as you are describing. I had cramps, back pain, sore boobs etc. I didn't even consider pregnancy as a possibility because I swore my period was going to start at any moment (and because we had been trying for so long I didn't think it was a possibility). My pregnancy test 4 days after my missed period showed a very faint positive. How many days late were you when you took the test? Most say they can detect even a few days before a missed period but not with as much accuracy. I would say if you took the test at a week late, it should have been able to detect pregnancy. But if you took it at only a day or two late, maybe not. Also, have you been on IVs at all this month? IV's have messed with my periods in the past, and I generally have very regular periods.
That's all I can think of for now! Let us know what you find out!
Autumn