The cause could be:
Electrical cause: you may have reversed the polarity of the signal or power supply conductors of the load cell.
Solution: check the connections by comparing the load cell data sheet with the wiring diagram in the instrument manual.
Mechanical cause: you may have reversed the direction of load of the load cell.
Solution: check the direction of load that delineates the direction in which the load cell has to work. You can find it indicated on the load cell by an arrow.