A light aircraft had an NMAC in the pattern with a Cessna at SNA.

Narrative

I was directed by tower on 126.80 (19R) to overfly the tower from the northwest at 1300 FT and to switch to 119.90 (19L). After crossing over 19R I contacted tower on 119.90 and was told to make a right 270 degree turn and to follow a Cessna turning downwind from a crosswind I followed the Cessna on a long downwind and was told by the tower that they would call my base. I lost sight of the Cessna after he turned base and continued down wind until the tower called my base. I completed my turn to base and to final when my passenger who is a pilot told me to turn left now!! There was an aircraft approaching from the right and below. I made a 360 deg turn to the left called the Tower to tell them what I was doing and resumed my approach. My question is where did the approaching aircraft come from?? Was he directed to 'switch to the left' from a straight in to 19R on 126.8 and not warned of my position? Did the controller on 19R not communicate with the controller on 19L? Also was I instructed to turn base too soon? I have been flying out of SNA for almost 30 years and have found the controlling to be excellent. Could I have mistaken my instructions form the tower??