Doctors again..

I'm so scared for my appointment on thursday.. Monday I went in got a blood test, I got 18.1 which is positive but they are worried it could of been a miscarriage I had a few days ago because I had my period but I tested a week ago (but i wasnt in any pain at all and didnt bleed heavy at all) and it didn't come up positive til after my period.? I don't understand. Now I'm hearing I could be only 5 weeks and it's also very likely the hormone isn't strong in my system because of a genetic thing. Ugh I'm so up and down about this I have no clue.. I don't wanna lose this baby