Please help!

My period is like clock work. My cycles are always 28-30 days long. I am now on cycle day 48 and no period signs. I took one pregnancy test and it came back negative, even though my period is a week late.

I was wondering if I could still be pregnant and when should I test again? My period still has not came. =( My partner and I use withdrawal, so I do understand that there is a chance I can be pregnant.