fresh SPR user need help for weird signal after injection

Hello! I am new user of BiacoreT200. I did a protein calibration test with CM5 chip on Monday and all data looks good. But on Tuesday morning, When i inject same protein sample, I saw all sudden negative peaks down to 0 after 80sec since the injection. after the injection was done, the curve from 4 channels go back to normal. I tried with desorbing and did not work. Now, if I inject even buffer without protein, I observed this weird data. Please help me. thanks!