Negative beta HCG, reassurance needed

Hi everyone,

20 days after intercourse and 3 days past my expected period, I did a quantitative beta hCG test, and the result was <0.10 mIU/ml. Three days later, I had a little brownish-red spotting, and the next day I had a heavy, bright red bleeding that lasted for 6 days. The blood was thick and heavy, with tiny clots.

How certain can I be that I’m not pregnant and that this was my period? Currently, I’m waiting for my next period, which hasn’t come yet. I haven’t had any sexual contact since then (about 60 days ago, after which I did the beta test).

Thanks in advance!