1st Day Of LMP 6/19/2015...July Never Got My Period...Aug 1st

1st day of LMP 6/19/2015...July never got my period...Aug 1st did a home pre test +positive (very light pink), Aug 4th went in to see the Dr. no sac was seen. I went back on Aug 6 for HCG level, returned for a second U/S on Aug 15 Dr saw a sac no fetal pole and said I was measuring at 4 weeks. Based on my last menstraul I would be so much further along. Does this mean I ovulated late or could I have ovulated with out a period? what are my chances of this being a successful pregnancy?