Unprotected sex 5 days ago and now spotted brownish color blood like the size of a quarter could it be implation bleeding?

Depends . After successful fertilization on the egg, implantation does occur around 5-7 days later. It may present as you described. Most preg tests are sensitive to show positive by the time of your missed cycle if you are pregnant. So, it really depends on the timing of the unprotected sex. There is usually a 20-22% chance of pregnancy per cycle.