The fuel trim numbers indicate the adjustment rerquired by the engine computer to keep the engine at the proper air/fuel ratio, about 14.7:1.
The short term fuel trim is an instantaneous value, based on engine load. The long term fuel trim is used to "zero out" the STFT after a period of adjustment of STFT.
As I stated, these numbers can offer clues as to what's happening. If you do not have, or are uncomfortable with, a scan tool, you'll need to find a savvy BMW indie to troubleshoot this issue.