No Gestational Sac

My period tends to be a little weird .. Some months it’s normal, others it can be a light two/three days. Last month was one of those months. I took 4 tests all positive. The two digital came up positive in less than a minute. I went to get an elective ultrasound to see how far along I might be .. Based on my last “period” I would be about 4 weeks 4 days. During the ultrasound, the sonographer told me the lining was thick but there was no gestational sac yet.

This could mean, I could get my period or I could possibly have not implanted yet? Has anyone had this happen before? What was the outcome?