Did you just start having your period in the past few years (maybe 5 or less years)? For the first few years, my period was all over the place, black brown, heavy light, long short. You can go on birth control to regulate your period. I use NuvaRing, so much easier to track and maintain than a pill. This will also obviously help with your pregnancy risks. You're not pregnant right now, but once you get checked for the items listed above, you will know for sure anyways. Best of Luck!
Well, if you are sexually active, then you do need to take a pregnancy test. Just to rule it out. Do you use protection? Bleeding more than 7 days is not good for you. I think you should go for a pelvic exam and discuss this. Maybe the pill would be a good option for you to help with birth control AND to regulate your period?