Discrepancy regarding stamp

[rickybest]

I entered in the U.S on B2 visa on 8th Nov.2013. I got stamp on my passport that I am admitted till 11 may 2014. I left the U.S on 9th May 2014. I did not get any i-94 form. I checked I-94 form online which says I was admitted till 7th may. Does that mean I overstayed in the U.S although stamp on my passport says I can stay till 11th may?

I didn't want to overstay but due to this discrepancy I got trapped.
I would really appreciate you help.

Thank you.