Pregnancy Blood Test

So I've been having all the signs of being pregnant, took a test which I believe came up as a false positive. So I made a Dr apt and they called me yesterday and said that my hcg levels were low but a hormone was present so I have to go back this Friday to take another one. I'm not late yet, usually I start anywhere from the 14th-19th every month. Has this happened to anyone else with a blood test?