CONFUSED (Implatation bleeding/spotting/brown blood)

Hi! Please help me, I am really confused.
Last August 10, me and my boyfriend have unprotected sex, it is a week before my period. Three days after I got a terrible headache. Days before my period will start, I started to have nausea, and I vomit a lot. I thought it was just stress and over thinking, On August 18, I got my period, it was light and almost did not cost me to wear pads. Worried, I bought a Pregnancy test, and tested negative. However, the bleeding lasted for 3 days. I started to feel bloated and I keep on belching. It was too early to test so I tested again a week after on the 25th, and got a negative result. So I looked for help and addressed my concern and said, the best result for homePTs would be 21 days after sex. So I did waited for another week, and tested on Sept.1, I got a negative and a faint line on the other. They said it was a positive, so then I should wait for 2-3 days to see a clearer line. I waited for 12 days, and got the same result. It was so confusing!

My expected period is Sept. 15, and I got my period, but it was so light, (in cases I did not wear any pads) and it is brown in color, it is accompanied with a pain in my lower back. Sometimes it is plenty, sometimes it is just in streak forms.

It confuses me a lot, assuming that if I am already pregnant, why did I got my period this month? Can implantation bleeding occur after a month? And if I am not pregnant, what does this brown blood mean? And when can I take another pregnancy test for accurate result? 🙁