Help! I'm bleeding & idk what to do!

I'm 23 and have had the Implanon implant in my arm since October 2016. I have had a pretty regular period just like clockwork since April/May of this year lasting 5ish days generally.

I just had my period from the 14th-16th of September (only 3 days) but I have been bleeding for past two days (25th&26th& now 27th). The first day was dark almost brown old type blood, but after that it has been very light red/pink.

I have been very stressed out, and my boyfriend and I have also had quite a lot of sex the past 5 days. I have been feeling abdominal and back pain along with cramping, dizziness and fatigue but no fever or vaginal pain/burning.

What does this mean?

Do I need to go to the doctor?