Had sex day of spotting with blighted ovum preg. Heavy miscarriage bleeding next day. Light bleeding 2 more days. Could that sex caused another pregnancy?

Probably not. It is unlikely if you have completely expelled the previous pregnancy that was a blighted ovum as Mother Nature will not reset the pituitary gland for another ovulation until the HCG levels drop from an existing pregnancy.
Lower risk. While the risk is quite a bit lower that it caused another pregnancy because you were already pregnant, we can't completely rule out the chance you may have ovulated around that time.I would be cognizant of any pregnancy symptoms in 4-6 weeks although it could take that long or longer before your cycle is "normal" again after miscarrying.Can check preg test around 5-6 weeks too. Good luck!