a friend of mine had that, she is 16 too. she went to the gyne and they told her to start using birth control as medicine, she is a virgin and doesnt have sex yet.
the docs usually tell me that after your first baby your period should get more regular, but if it isn't normal to have a long period and it doesn't go away in over 3 to 4 weeks I would suggest seeing a doctor, this isn't uncommon though, it happens to a lot of teens (it's just really annoying). look it up on google too