HCG levels question

So I got my positive opk on CD 21 (mar 28) and a faint positive on Sunday evening with a darker line Monday morning and even darker Monday evening. This morning however, was a little lighter. I had a blood test and it shows my HCG at 15- they explained it can be a healthy pregnancy or a chemical pregnancy we just have to test again tomorrow and see where we are at and if it is doubling. Could my levels be low from ovulating so late? I was so excited to get a positive from my first round of clomid and now I’m stressing myself out.