Hi there, and thanks for the ask!
It’s certainly possible that preferring they/them pronouns could be a sign that you’re not cis after all, and therefore could be some form of nonbinary, including agender. Everyone starts questioning in their own way.
It’s important to remember that pronouns (like all facets of presentation) are separate from identity – a person of any gender (or lack thereof) can use any pronoun set. So pronouns don’t determine identity. Plus, there are lots of different nonbinary identity labels, and agender is just one of them.
I think it might help to try to consider why you felt so happy when that person used they/them for you. Maybe you were happy because you appreciated that they didn’t assume that they knew your gender based on your posts (which would likely be really rude and cisnormative), maybe you felt happy because you’re discovering that they/them pronouns suit you better, maybe you’re discovering you’re nonbinary, maybe some combination of those, or maybe there was a totally different reason. There are lots of potential explanations for your feelings!
Remember that it’s okay to be questioning. Take your time and think about who you are, how you feel, and the labels you use. It’s also okay to change your pronouns and/or labels, as many times as you need to, along your journey of self-discovery. If you want to talk with more people about what it feels like to be agender in particular, a great way to do that is to join us in chat.
Hope this helps!