Hello and welcome,
All types of contraception
have a small chance of failure, and the ones with least or absent chances of failure are vasectomy
and tubectomy. I would advise you to visit your obstetrician/gynecologist and get yourself checked, although 3 weeks is too early, I would recommend a check up planned for the end of the 4th week for better chances of detecting a pregnancy
In the case of a pregnancy the obstetrician can advice you accordingly, as this is a case of failure of contraception.
In case of an absent pregnancy, the doctor can help relieve you of your symptoms and also schedule another check up to be on the safer side.
I hope I have succeeded in answering your questions, you could write back to us anytime for further clarifications.