Worst nightmare!

We have been trying for 16 months and I finally got my bfp this Monday..I was over the moon...Took digital clearblue and i got 2-3 weeks pregnant.So on Friday evening I went to my gynae and did an ultrasound...I should be around 4weeks but unfortunately doc cant see the sac.He said my endometrial lining is thick but otherwise nothing.He said maybe still early and asked me to do a hpt again.The worse part? He couldn't see any positive line and if anythg it was very faint.I was heartbroken.Today i took a hpt again with my afternoon pee and i got a pretty clear line..Im confused and scared..