I Am A 25 And Recently Had An Ultrasound And

I am a 25 and recently had an ultrasound and found that my endometrium is 13mm in thickness.. I took a pregnancy test and it was negative. And I am currently on the Ortho evra patch. The ultrasound was taken 12 days after my patch free week.. can you please explain how this could be? Or why the thickness is 13mm. The u/s tech said I Could be pregnant