multiple blood tests to confirm pregnancy?

Robyn
Two weeks ago, I had an appointment with my doctor because I wanted to get checked out for PCOS. It had been 60 days since my last period, but kept getting negative pregnancy tests. She ran a pregnancy blood test along with the other tests just in case, and said I had a 4. That night, I took a test just to check and it said positive! Three more came back positive too. 
One of the tests they did was messed up by the lab, so last week I went back for more bloodwork, and they did another test for pregnancy.  Yesterday a nurse called and said they want to do a third blood test, just to confirm and compare numbers. They're estimating that if I am pregnant, it's about 4 weeks, so too early to show on an ultrasound. Now I'm worried though.. why aren't they saying "yes you are pregnant"? Has anyone had this happen before? They won't say if my numbers went up enough or not. I miscarried at 9 weeks in February and don't want to go through this again, and I'm freaking out.