I have read of many women who have low hCG in early pregnancy only to go on to have healthy bubs. It could just be that you O'd a day or two later than you expected, it is very early days still. Your brown spotting sounds like an implantation bleed, these can either be brown or pink, but never bright red (so I read).

Best of luck, I really hope this bub snugs in nice and tightly for you .