Pregnant or not

So I am very confused this is my first time trying to get pregnant and I have a few things I am wondering about. I missed my period in December. It’s been 29 days that my period been late. I have been taking pregnancy test back to back and all of them are saying negative. I spotted for about 2 to 4 days maybe less on December 26. I haven’t took another pregnancy test since December. I also went to the doctor they also said i wasn’t pregnant. Should I take one again? What should I do ? What’s good advice ? Do any one knows the causes ?