vicodin and pregnancy test

Hello,
I am about a week late. I have some cramping which to me usually indicates that my period is coming. However, I've been having cramps for about four days now and nothing. I recently took a pregnancy test and it showed up positive. Now, my period is irregular. I usually get it for 2-3 days the begining of the month and then for at least 1 day at the end of the month. I had also stopped taking VICODIN for my bad back about a week before I took the test. Could the pain killers have had an effect on my results? Why am I cramping? Please let me know. Thanks.