Unless this therapist is a complete loser, she saw something in you. That does not mean she interpreted it right. But my guess is she noticed something in your nonverbals that made her think autism. It's actually quite hard to understand what nonverbal signals you yourself send out. If you lack feedback, you might think you behave in a manner you do not.

Now I'm not saying you have autism but yes you should ask why she thinks that. Don't just throw it away.

And yes it is very possible to have autism go undetected into adulthood.

In Sweden you cannot be formally diagnosed unless you go through 2-3 days of intense testing by a team of experts. Unless you pay up, the waiting list is 1-2 years. So you really do not have to worry about being diagnosed at this point in time.