My period was six days late and I've been having intense pain and I passed a rather large clump of grey tissue. Could this be an early miscarriage?

Possibly. Most pregnancy losses present this way. No matter, you should continue your activity as regular and see your gynecologist.
Miscariage. Yes probably if tissue does not disintegrate on pressure as a blood clot would. Needs to see a doc to insure uterus is really empty. Any remaining tissue need be removed to prevent infection.