I don't know how to feel

I (29f) have always been anxious. Highly so. Chronically, even. Sometimes I may just aim to please, especially if I care about the person's feelings.

The other day, I was doing what I'm used to and reassuring someone I know how I'd do something, despite not liking the subject. I was then told by her that I didn't need to feel obligated to. This shocked me. I have heard this response before, but I knew that this person meant it and wouldn't later guilt-trip me. She told me to actually think about what I wanted to do, and I felt stumped.

I later decided to not do the thing I didn't actually want to, but I've not felt right since realizing I don't actually HAVE to do things I don't want to. At least not with her. But that thought of not having to, I just felt suddenly blank. I wasn't worrying, fretting, or nervous. I wasn't even excited. I don't know why. It's like when the obligation wasn't there, I felt lost.

I went to do my typical volunteering today, but I still felt a bit off. I tried to smile and look friendly, but I just felt off. I don't understand why. Has anyone had this experience before, or can advise me?