If the negatives were taken with FMU and were taken after you got the positives then chances are you had a chemical pregnancy. If they were taken later in the day then take another test with FMU. If it's negative then you most likely had a chemical pregnancy.
Also when did you get the positives? The best timeto test is with the first morning urine. I'd you're really early in your pregnancy it might nit show up later in the day yet
You could have had a chemical pregnancy. If you can, go to your doctor and get a blood test for pregnancy. It won't give you false information.
I would say yes. False negatives happen, false positives RARELY ever happen.