Am I pregnant?

I had sex for one of the first times on the 6th of this month. My periods are usually irregular. I may be a week late or a month late. So it’s usually a guessing game. Well, I was supposed to start the 20th, which I did. However, I had a very tiny bit of bleeding on the 10th none on the 11th, so I took a test and it was negative. I thought maybe since I was spotting, I could be pregnant. Then I started spotting again a few days later. It was continuous spotting here and there until my period started. It lasted a few days(about 3) of a pretty hefty flow. Then slowed down and ended. It surprised me because I don’t really spot or start on time. I don’t know if I have a problem or if this is a sign of implantation bleeding. I also took a test today and it was negative. I would be about 3 weeks and a day pregnant if I am. The test is still negative. I am also 18 and just started having sex about the start of this month. In a month, I may have had sex 3 times.