While I can't claim to have watched the game, looking at the boxscore, several things jump out:
First, the Heat were simply out-rebounded. Indy posted 49 rebounds (14 offensive) to Miami's measly 33 (9 offensive). It's easy to say a team needs to rebound better, but a minus 16 rebound differential is almost impossible to overcome. At least for tonight, Bosh posted only TWO rebounds. He's almost a 7 footer, so two rebounds is simply unacceptable.
Second, it certainly didn't hurt that the Pacers shot well from three, at 10 for 18 (56%).
And finally, it seems that the Heat weren't playing their style tonight. Watch any Sportscenter from this year and you're likely to see a fast break dunk by Lebron or Wade, or some other form of "easy" points. Tonight? Four fast break points. It's hard to say if the Pacers simply matched up well against the fast break, or if Miami just got away from that style of play that has been successful.
While two losses in a row (especially with one coming against OKC) is worrisome, the Heat have time to rebound--pun intended.