Menstrual Cycle as a teenager

I got my period in June, but I skipped my period in July. I had unprotected sex but did a pregnancy test and it came back negative. I drank a couple of sips of soda I hope that didn't affect it. I just got my period the 1st of August. I have my period for 4 days. But these couple of days have been a little different. Usually the 1st day will be light, 2nd day will be heavy, 3rd day will be a little heavy then light. But the 1st day was heavy, 2nd day was light, 3rd day tmi saw a little bit of blood on my pad. When I went to clean my blood was like light brown that usually doesn't happen to me and I didn't see any blood in my urine. It's only been the 3rd day and I saw light brown blood. Its not even the 4th day yet that usually happens on the 4th day. I'm a teenager I had unprotected sex but I took a pregnancy test it came back negative. Does that cause to mess up my cycle? And make it irrgular? Freshman year of high school I skipped my period for like 5 months. Ever since I think its been regular. But whenever I would skip it would be of stress. Can unprotected sex mess up your cycle even though your a teenager and the test came back negitive?