If she wasn’t in a bad mood prior to you asking her about becoming exclusive, there is a good chance that your request to become exclusive with her is the source of her bad mood.
She is still not willing to admit that to you because she is not sure about what to do with this relationship at this point.
Basically, she is still trying to buy time. Until she figures out what to do with this relationship, she will most likely want to continue texting you.
However, it may not be to your benefit to just hang around in the hopes that she figures out what she wants to do with you.
You could respond to her message by asking her if she would like to take a temporary break from the friends with benefits relationship in order for her to clear her head.
The thought that you may spend that break possibly hooking up with other girls may plant a sense of urgency in her head that could make her feel the need to figure out where to take her relationship with you a lot quicker.