Tiagoamarante:
OK.
I know that whenever there is something above 0 in the serial port, i know that it is a sensor and that the sensor always sends 34 bytes every second.
Yes, but you don't know whether they've ARRIVED YET.
Your Arduino runs very quickly compared to the time taken to transmit 34 bytes over a serial link, and it is perfectly capable of spotting the first byte and processing it before the subsequent bytes have finished transmission.