Hi Renan,
1. Please check how many bits of your MCU is. Is "signed short" represent 16 bits in your MCU?
2. Please make sure you did not wirte weird values to OFSX, OFSY, OFSZ registers by accident.
3. Please make sure you did not enable self_test by accident. It looks not since you write 0x0B to register 0x31. But please double confirm.
4. ADXL345 needs turn-on time, it is 1.1ms + 1/output data rate, so please check the valid output after the first several samples.
BR
Neil