I had spotting and cramping at 6-8 weeks nearly every day, then a LARGE bleed ( running down my legs) twice at 13 weeks, no cause ever found and no bleeding since, so anything can happen and babies still survive. Sounds like yours is doing well- the HB is fine too so try not to stress about that.
Take it easy and rest. I took 4 weeks off work in total and took it really easy.
If the baby is measuring on dates and good heartbeat I wouldn't bother with HCGs- the scan is more of a reassurance.
Also, it doesn't sound like a 'traditional' implantation bleed, more like from something like a subchorionic haemorrhage as pp's have mentioned.

Try not to stress (impossible I know) xx