Im 6 weeks and nothing seen on sonogram!!

So on dec 12 i had a miscarriage my levels went back to 0 . So later on this month jan 10 i got a positive test becouse my periods come on the 9 so right now im 16 days late , base on my lmp im sopposed to be 6 weeks by now i went to my obg today and he couldnt see nothing on my uterus but my hcg levels are 1,250 so i have to go back on wed,thur,friday and on monday they will let me know whats going on but im super strees what could it be!!'help any body has expirience that!