Why do I have a + pregnancy test but nothing showing on US?

Bori

I haven’t had a period since February 5 and getting a little concerned considering I went to the doctor to get an ultrasound and nothing was there not even a sack.  just very confused on why this is happening because I usually miscarry but in this case the lines on a test are getting darker and darker but I should already be eight weeks considering I haven’t had a period and almost 2 months… What exactly do you think could be the issue? And what could I possibly do in this situation?