possible miscarriage?

i have just missed my period and have done four home pregnancy test which have all shown a positive result. 2 days ago during intercourse with my partner i started to bleed it was slightly heavy at first and has been stopping and coming since. on my first 2 pregnancy test that i took the day of my missed period, the second line was very faint- that was 4 days ago now. i went to the early pregnancy unit yesterday the morning after the bleeding and they done a pregnancy test that showed a negative result, but said to come back next week as i could be too early in my pregnancy to get a positive result. i took it upon myself to do another pregnancy test this morning and again it was a positive result, only this time the second line was alot more viable and darker in color. i have had slight cramping and the bleeding is only light to moderate and there is no passing of clots or tissue.. any ideas as to what this could be would be very helpful?????