Late period,negative result on home pregnancy test.?

I was due on Monday 25th, but I still haven't come on. My periods are usually very regimented and have always been on time. The only time I missed my period previously was when I was pregnant. Yesterday and today I have had a very small amount of brown sticky blood on the end of a tampon. I thought this was possibly implantation bleeding as I experienced this in my last pregnancy. I experienced all the usual period symptoms of cramps a week prior to my expected period, yet it has still not arrived. I even went to the doctors yesterday and they said my temperature was a little high, which again I thought was attributed to early pregnancy; the doctors did not test for pregnancy however, as I was visiting for another ailment, my raised temperature has only just clicked as a pregnancy symptom. However, I tested today and the negative was very clear and instant, but I cannot think of any other explanation as to why I don't have my period. Any help would be appreciated!