Late on my period negative pregnancy test


So i been ttc since agust with this <a href="">ovulation app</a>. And no luck same regular periods. 31 cycle 5 day length period normal . Last day of period was on 11/24/14 and we are now on dec. 30 2014.

Im almost 2 weeks late. Took a preg. Test at the drs. And was neg. My dr told me to come back in 1 week and check back with her to retest. Does avulation take long? Or my hcg hormone to find out???