If they were urine tests then chances are they could have been wrong. There are many,many cases of fasle negatives when it comes to urine tests. I know with my first son I had to wait an extra two weeks after I missed my period before I urine test worked for me. I also have known people who never got a + urine test even though they were 2 and 3 months pregnant. So dont count pregnancy out just yet. If there is a chance in other words if you are sexually active then go get a blood test by your doctor.
Now there are other reasons. Such as stress or being sick can throw off your cycle. When a cycle gets thrown off it tends to effect you in other ways as well. Such as more intense PMS or cramps.
Also PMS signs can be identical to pregnancy signs.
DS 13 yr
DS 8 yr (3+ yrs ttc)
TTC #3 since 01/02