What Causes The Fluctuation In Pregnancy Test Results?

I have taken 2 pregnancy tests 2 days ago. Both are positive, and had very visible results on the stick. I had gone to the hospital after hours months ago and took 2 tests then, and the nurse said they were positive but when I took one at home it said negative. I never saw the tests that the nurse had me take but something was mentioned they may have been ovulation tests by accident. But as I had said I took a home one then and it was negative, and took 2 a couple days ago and they were very much positive within seconds. Is there a chance I might not be pregnant this time? Is it possible the nurse didn't know what she was doing and I am now?