ACR ON SHORT FINAL ISSUED INSTRUCTIONS TO TURN L AND S BECAUSE OF AN ACFT EXITING THE RWY AND AN ACFT IN POS DEPARTING.

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DARK; HIGH OVCST NIGHT. VISUAL APCH TO RWY 27 IN HOUSTON. RWY 15L WAS CLOSED AND THEY WERE DEPARTING AND ARRIVING RWY 27 --VERY UNUSUAL FOR IAH. WE WERE CLRED TO LAND. THERE WAS A COMPANY ACFT DOWN THE RWY AND TWR WAS PRODDING HIM TO EXIT THE HIGH SPD. PROB WAS; THERE WAS ALSO AN ACFT ON THE RWY THRESHOLD; BEGINNING TKOF ROLL; AND WE WERE SHORT FINAL AT 300 FT. THERE WAS NO WAY IT WOULD FIT. NEITHER MYSELF NOR THE FO REMEMBER HEARING THE TWR CLR ANYONE INTO POS; OR CLR THEM FOR TKOF. WHEN WE CONFIRMED OUR LNDG CLRNC; THE CTLR SEEMED A BIT CONFUSED AND SAID 'CANCEL YOUR TKOF CLRNC.' THEN HE SAID 'CANCEL YOUR APCH CLRNC;' AND GAVE US A L TURN TO A SOUTHERLY HDG. HE NEVER DID SAY 'GO AROUND;' WHICH WAS WHAT WE WERE EXPECTING. WE EXECUTED THE GAR AND BEGAN THE TURN; AMID ALL THE CONFUSION. I (THE CAPT) WAS FLYING; AND THE FO WAS QUITE BUSY GRABBING GEAR; FLAPS; HDG KNOB; HDG SELECT; LISTENING BOTH TO MY CALLOUTS AND THE TWR INSTRUCTIONS. I'M NOT SURE IF WE WERE ASSIGNED 2000 FT OR NOT. IT COULD BE THAT WE WEREN'T ASSIGNED IT; OR THAT THE RADIO CALL CAME RIGHT AS I CALLED FOR MORE FLAP RETRACTION. WE WERE ACCELERATING AND CLBING RAPIDLY. THE FO ASKED THE CTLR WHAT ALT HE WANTED; AS WE WERE GOING THROUGH 2500 FT; AND HE SAID 'WELL; I WANTED 2000 FT.' WE STOPPED AT 2600 FT; AND WENT BACK DOWN. I REALLY DON'T THINK THE CTLR KNEW THERE WAS AN ACFT ON THE TKOF ROLL IN FRONT OF US. THIS; AND THE AMBIGUOUS CLRNCS ISSUED TO US CONTRIBUTED TO A CONFUSED STATE AND HIGH WORKLOAD ON THE PART OF THE CTLR; AND TO BOTH OF US ON THE FLC.