21 y/o sexually active girl. My period was late one day, was first brown, and is now clumpy and red. Should I be worried? Pregnancy test neg 2 days ago

No worry. If you are having your period, as it seems, you are not pregnant. If period is less than your usual, you may repeat the test in 4-5 days. If you do not wish to be pregnant, use contraception all the time, every time. You may wish to consider an implanted contraceptive. Always practice safe sex. Get vaccine for HPV.
Irregular bleed. It could be that you had a cycle where you didn't ovulate, but this could also be implantation bleeding from a new pregnancy. It maybe too early to have a positive pregnancy test with the brand name you are using. If you're not using protection, this might be a good time to see your physician for birth control and get a serum pregnancy test.