I'm in quite a weird situation here. I've recently been fuck buddies with my ex, we broke up a few years ago. A lot of crap happened in between that, but we dealt with it and became friends, and in the last 2 months or so we've become sexually active together. Throughout we have both maintained that we don't want to be in a relationship, and shes even said several times that she'd like to spend the next year single.

Today however, something seems to have changed. She was down about something so we were talking on the phone, I cheered her up, and she said that she finds it very hard to tell me when shes upset, because she doesn't want me to get annoyed with her, or think that shes attention seeking. I re-assured her by saying that we're friends and that she can talk to me about anything.

After I said that, she seemed to go them quiet and distant and I'm not entirely sure why. I know she hates being questioned so I don't want to ask her about it. Any idea what has changed?

She feels herself getting close too you, and she doesn't want a relationship nor too get hurt again.

"You can talk to me about everything" isn't something you tell her, it's something she should feel when she talks to you.
I don't know why she's distant and quiet, but i wouldn't chase her.

Obviously since you broke up and have been split up for a long amount of time, a relationship is not the answer. She just wants sex. Nothing more.