1 cm, perfectly circular bruise on top of foot. Very small pinpoint at center. No swelling, redness, raising, etc. What was I bit by? Cause for worry?

Varies. There are a number of possibilities, among the more common and sometimes concerning can be a spider bite, particularly a brown recluse spider. You should seek attention if the area becomes dark appearing or shows any sign of infection such as redness or swelling; or if the size of the area is increasing.