Vomiting or spitting up in sleep... help!

My baby never spits up and has never thrown up before so I’m a little nervous...

She woke up at 3am for her usual overnight feed. I changed her, nursed her and put her back in bed awake as I always do. This morning when I heard her wake up I went to get her, and there was a big wet patch in her crib. Her arm was wet too. She seemed totally normal otherwise- not even annoyed or distressed to be lying by this cold damp patch.

How can I tell if it was spit-up or vomit? Is there a difference at this stage? And should I be worried? Like I said, she’s never thrown up before and never really spit up at all. She doesn’t have a fever, I’ve nursed her again and she seems totally normal. She’s 4 months.