urgent!!! need advice please....

right now im in china, my I-751 is pending, my two year green card has expired. I never received NOA (receipt) for I751, so before i traveled overseas, i had immigration stamp temporary I-551 on my passport, is that enough to travel back to US? The expired green card + I -551 stamp on my passport?

Because when i was checking in at the US airport on the day back to china, the airline lady who checked me in said i might have problem come back in coz of my expired green card, i showed her the I-551 stamp on my passport, she says she's not sure, that worries me alot. please advise, thank you.