Help me!

Right, I am a week late for my period. 
I went to the doctors last week because I am being treated for endometriosis and I was in agony and was being sick. They took a test and it was negative. However whenever I take tests they either come up negative or negative and half positive.  It's been a week and I still haven't come on and I'm being sick, upset stomach, sever pains and headaches. Is it worth taking another pregnancy test or is it normal for this to happen during endometriosis?