Budgie vomited

I took my budgie outside with her cage and removed the bottom of the cage so she could nibble on grass in the back yard. I have done this many times before and she was fine afterwards. But today after eating grass, she started vomiting up whole seeds with this yellow-ish sticky stuff.

I've researched the difference between regurgitating and vomiting and this was definitely vomiting. I don't know what happened! She has never done this before and she was shaking her head while vomiting.
Right now she seems ok. She isn't fluffed up and she's active. But what could have caused her vomiting?

Thank you