Help please pregnant but no sac???

I found out I was pregnant a couple of days ago I took 4 test and they all came but positive. This morning I woke up with really bad cramping I couldn't even walk than I felt a pop in my stomach about a hour later. I had a doctors appointment today and they did a test which confirmed in was pregnant but when they did an ultrasound they couldn't find a sac. What does this mean?? Am I having a miscarriage???

I'm still having cramps now they come and go I'm not bleeding through so I dont know what's wrong. Has anyone had this happen to them help please.

And I'm still having all the pregnancy symptoms cravings and everything.