While I obviously can't say for sure whether or not you have parts or DDNOS or anything, it certainly sounds very possible to me. It definitely seems like this was a "sighting" of a little, and not a little too much imagination. I used to have extremely similar experiences with Cassie before I was fully aware that she was "there"; and I had similar experiences again before I became aware of Lynn.
From reading other posts of your's and reading this one, I would say that DDNOS/DID is a very strong possibility with you, and that you probably have a little part in you that's still reacting to any childhood trauma you experienced. I know it's hard to accept, believe, or even think it's possible; denial is especially common in people who experience dissociation because the whole reason they're dissociating is to "stop" that trauma from happening by removing it from memory, hiding it, making excuses for it, etc. But you seem like you're trying to keep an open mind, that's good, don't lose that. It can be hard to keep an open mind, especially when you don't like what you hear. Try to keep in mind that anything is possible, and that being diagnosed isn't something to fear. It can help you help yourself and figure out pathways to heal.
Have you tried helping communication between your dissociated parts? Perhaps try talking to them or writing notes to them. Make sure you feel as safe, relaxed, and calm as possible before trying contact. Surround yourself with things that make you feel safe and comforted. You could even try asking about the possible little part that you noticed and see if anyone "answers". Let your parts know that it's safe, they're safe and can come forward, that you only want to get to know them better and help them. Sometimes communication can improve from nothing to something with encouragement and patience.
I'm sorry you're feeling discouraged. -hugs if needed- But when you think about it, this healing process wasn't going to be "simple" no matter what. PTSD, no matter how "little", is still quite the healing process and deals with a lot of the same symptoms and steps to healing as DDNOS/DID. Besides, wouldn't it be better to figure out the right treatment for you before you go into treatment for something else that's not working on the whole issue? It'll be better for you in the long run to figure out if you have DDNOS/DID now. I think you are ready to start therapy, because either way it will help you. Whether it helps you heal through therapy, or whether it helps you heal through bringing parts to your awareness, it'll be helpful to you.
I hope that therapy goes well for you, and I hope that things start becoming clearer for you and that you find a direction soon. -more hugs if wanted-