Started having brown spotting after sex about three days ago. Dr said that it was fine and would last a couple of days. Today it has seemed to get heavier but I only notice it when I wipe or go to the bathroom. I am eight weeks

It likely is from the sex. You could try refraining from sex for a week and see if it continues on its own. Bright red bleeding, especially if accompanied with cramping, would be more of a concern. Certainly check back with your doctor if you continue to be unsure. The peace of mind is worth it.