NMAC WHEN PLT OF C152; ACFT X; FAILS TO STAY ON DOWNWIND LEG TO AWAIT CTLR'S TURN TO BASE FOR TFC; ACFT Y.

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TWR INSTRUCTED ME TO CONTINUE A R DOWNWIND FOR RWY 15. HE WAS TO CALL THE TURN TO BASE. OPS AT THE ARPT WERE CONGESTED. THE CONDITIONS WERE CLR; WITH DAYLIGHT SLOWLY APCHING DUSK. I BELIEVE THAT THE TWR HAD INSTRUCTED ME TO FOLLOW TFC Y THAT WAS ON FINAL APCH. DUE TO THE CONGESTED NATURE OF THE FREQ; I DID NOT REPEAT THE COMMAND BACK TO THE TWR. I TURNED TO FINAL WHEN THE EXPECTED TFC Y WAS ABEAM ME. A FEW MOMENTS LATER; ANOTHER ACFT; ACFT Z; WAS RAPIDLY CONVERGING UPON ME. THERE WAS NO ACTUAL DANGER OF COLLISION; BECAUSE I HAD SPOTTED THIS ADDITIONAL TFC AND WAS ALREADY TURNING AWAY. HOWEVER WE MAY HAVE APCHED TO WITHIN 500 FT. THE OTHER ACFT Z ASKED THE TWR ABOUT ME. I RESPONDED TO THE TWR'S QUERY WITH A READBACK OF MY INSTRUCTIONS. THE TWR SAID THAT THE INSTRUCTION I HAD FOLLOWED 'WAS NOT FOR ME.' THE TWR THEN GAVE ME OTHER VECTORS TO FINAL. I LANDED WITHOUT INCIDENT. WHEN A PLT BELIEVES THAT HE UNDERSTANDS AN INSTRUCTION; WHAT IS THE RIGHT THING TO DO WHEN OPS ARE AT A HIGH ENOUGH LEVEL TO MAKE THE CASUAL REPEATING OF INSTRUCTIONS IMPRACTICAL?