Can implantation bleeding occur after your missed menstrual cycle?

I have no idea I had unprotected sex about 3 to 4 weeks ago I was about 13 days late on my period I started today but it is really red with a little tiny bit of brown on some places also my breasts are kind of tender I get head aches and also I was having mild cramping could It be implantation bleeding it was heavier when it first starded when I woke up and now that its later on in the day it slowed down. But I don't understand how long it would take for implantation to accur like I said its been up to 3 or 4 weeks