2nd HCG level on Monday - thoughts?

I had a BFP 10 days after receiving a +OPK. The line was VERY faint, almost not there at all. Does anyone know when I would have likely conceived? We had sex the day before, 2 days after and on the day of my +OPK. I went to my OB and my HCG level was only 19 11 days after +OPK. I have to go back Monday to get another level done and I am extremely nervous. Is 19 low?