False negative

How likely is it that I had a false negative? My hormones have been all over the shop. My partners sister has just had a baby and I cried wishing it was mine. I had bleeding a week before my period was due but it was ridiculously light for me. Tested on the day before my period was due and negative.

Could I have tested too early? And what I experienced was implantation bleeding? And if we have sex on my estimated ovulation day but we already are will this harm the baby?

I’m already taking folic acid just in case.