Had sex about a week after my period..missed bc for 2 days did not double up...got cum inside had breakthrough bleeding could i be pregnant?

Sure, if missed pill: A urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a "period". If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and thus lead to a repeat test. If one gets her normal regular "periods", then she is not pregnant, and a test is not needed.

Potentially: Would take a home pregnancy test if your period is late.