Dark churning?

I ticked back over the last few weeks, months, to think how he and I might be connected. But there was nothing, just a dark churning in my mind, he does know me, I thought, He knows who i am.

When I looked up the dictionary, churning meant agitate a liquid to give butter....
but in this context did the writer means dark ideas agitated in the character's head?

Thank you in Advance,