Pregnancy at risk?

So my last period was May 19th, but I know that I ovulated June7th, and my husband and I had plenty of sex during that time.  I've gotten the positive pregnancy test and saw the doctor today.  The only symptoms I'm having are tender boobs, and some light cramping and some lower back aches.  I miscarried this past October, so they did an ultrasound today since I am 6-7weeks along if you count since my LMP.  However they only saw a sac.  They have me coming back at the end of this mo th, and said it may just be too early.  I also took a clear blue digital test today that shows I am 2-3 weeks along... Is there a chance my pregnancy has stopped?  I have had no spotting or bleeding.  Just nervous since we lost a baby in October, and are really wanting this one to work. (Would be our first child.). Thanks for any honest advice!