How worried should i be if implantation bleeding looks heavy?

Either way: Yes it could go either way, but there is no reason to truly worry since there is nothing to do but wait and retest yourself. I know thats easier said then done, and i hope it works out.

Likely breakthrough: It's almost never implantation bleeding, that is largely an internet myth. If - a big if - you actually get implantation bleed it woudl be very light and about 6-7 days after ovulation, or about a week before your expected period. So, i wouldn't worry.