Have you done a preg test? Some women can have pregnancys where they have no symptoms.. Btu as soon as they find out they kick in!
If you were 3 weeks late and you are or were a wee bit sore down there, even though you had a peroid I would suggest a preg test, if negaitve go to your Dr. It could be a viatmin defiancy or something related to stress.
Failing that maybe you had a miscarriage and the after bleeding was the final bits.
If you then had a infection afterwards I would probaly say that you were pregnant.. And were gone 7 weeks, and if you have one after 7 weeks they tend to do a d&c to clear out any residue to make sure there is no infection..
Best talk to your doctor if you are really worried, but things like excessive excersie, stress and vitamin defiancys can all cause you to stopp yor peroid.
Best of luck