I just got RFE in the USCIS online status, now I'm nervous

I don't know what evidence they might be requesting, I have to wait for that letter.
__________________
GA Applicant-Date Sent:
10/23/12-Certified-Chicago Lock Box-Date delivered -
10/26/12
Date of I-797 C Notice of Action: -
10/30/12-Biometrics notice and appt -
11/13/13-12/06/12 Walk In Biometrics
11/20/12-RFE-
02/19/13-received on
03/07/13-sent again on
03/09/13-2ndRFE
04/18/13-delivered
05/17/13-Approved-06/05/13Date of EAD received -???