Pregnancy test 7 non existent at 5 weeks- why?

I’ve been taken some pregnancy for the past week, about 7 to be exact. 6 of them were positive, however the last 2 I took, 1 was faint like the others or darker than the 5th at least and the last was completely negative. I’ve had no cramps or bleeding and have had symptoms but now I’m worried because my tests have faded and now there’s nothing. Does anyone know why? Has anyone experienced this but still been pregnant? Thanks.