PA28 PLT IFR LNDG ASN HAS AN NMAC WITH A B737 RESULTING FROM CONFUSION OF ASSIGNED ALT.

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ON A WBOUND COURSE TO ASN (TALLADEGA) FROM THE ZTL CTLED AREA; I WAS TOLD BY ZTL TO RPT FIELD IN SIGHT AND DSND FROM CRUISE ALT OF 6000 FT MSL TO 5000 FT MSL AND HOLD. I WAS TOLD THERE WAS A B737 INBOUND TO ANNISTON WHICH WOULD CROSS MY PATH. I WAS THEN TOLD TO SWITCH TO BHM APCH. I RPTED IN WHO I WAS AND THAT I WAS DSNDING TO 5000 FT MSL AS INSTRUCTED BY ZTL. BHM APCH CAME BACK AND TOLD ME TO DSND TO 4000 FT MSL AND HOLD BECAUSE THERE WAS A B737 ENRTE. I CALLED BACK THAT I WOULD BE DSNDING TO 4000 FT MSL AS INSTRUCTED AND THAT I HAD THE B737 IN SIGHT. AFTER A FEW MOMENTS I NOTICED AS I WAS DSNDING THROUGH 4500 FT MSL TO 4000 FT MSL AS INSTRUCTED THAT I WAS HEADING STRAIGHT FOR THE PATH OF THE B737. I CALLED IN AND REQUESTED WHAT ALT THE B737 WAS AT; AND WAS TOLD HE WAS DSNDING THROUGH TO ANNISTON AND WAS ALREADY WITH ANNISTON RADIO. I ASKED FOR IMMEDIATE DEV L TO AVOID AND BEGAN TURNING AS THE CTLR FROM BHM APCH APPROVED. HE THEN ASKED ME WHAT MY ALT WAS AND I CALLED BACK 4300 FT MSL DSNDING TO 4000 FT MSL AS THAT WAS WHAT I HAD BEEN GIVEN AS MY ASSIGNED ALT. HE THEN; ANGRILY; CAME BACK TO TELL ME THAT I WAS SUPPOSED TO BE AT 5000 FT MSL AND WAS NOT CLRED FOR 4000 FT MSL. HE TOLD ME TO RETURN TO 5000 FT MSL IMMEDIATELY. I RETURNED TO 5000 FT MSL AS REQUESTED AT BEST CLB I COULD MANAGE. BY THIS TIME THE B737 AS WELL AS ANNISTON ARPT WERE BEHIND ME AND I WAS OVER MY DEST ASN (TALLADEGA) AT 5000 FT MSL. I CALLED FOR FURTHER INSTRUCTIONS AND WAS TOLD TO HOLD VISUALLY OVER ASN. I CALLED BACK FOR AN 'EXPECT FURTHER CLRNC' TIME ESTIMATE AND WAS NOT GIVEN ANY KIND OF CONCRETE TIMEFRAME. I FEEL THAT THE BHM CTLR EITHER MISTAKENLY GAVE ME 4000 FT MSL AS AN ASSIGNED ALT OR HE DID NOT HEAR MY REPLY TO WHAT I THOUGHT WAS A CLRNC TO 4000 FT MSL. HE THEN BECAME VINDICTIVE TOWARDS ME TILL I CANCELED IFR OVER ASN (TALLADEGA).