What do you guys think?

So I’m 5 weeks pregnant. And I just went in for an ultrasound, but they couldn’t see anything. Right now there telling me they can’t tell me if I’m pregnant or not, but I’ve been getting blood work done every 2 day’s and it’s been doubling. Last week my hcg was 5.0, 4 days after 34, a day after that 54, and I just got it done again today. What could this mean? I’m not understanding why they can’t see anything. My son was seen at 4 weeks pregnant ...?