Just read your first post, and I too had difficulty understanding what you wrote. I assume it was because you were so upset. I get it.
Your son is not doing anything wrong. And neither are you. You are doing the best you can, and you are a great mom for being there for him...
All she is trying to tell you is that it seems that he is having difficulty to change from one thing to another and needs to be evaluated to rule out Autism.
And mommy, telling him, "good job" is great encouragement. Don't stop doing that.. He needs to know he is doing good. I really don't think that is what she meant by telling you he has a problem.
My son is Autistic and its of no fault of his own or mine. He too knows his ABC, counts to 20 and backwards, he can read numbers and letters and he is only 3. He had speech delay and is also in a speech and OT class in a public Elementary school.
But there are other little things that he does throughout the day, like continuously opening and closing doors and gets fixated on anything that moves fast. Be it a toy or a car passing or the wind blowing in the trees...
The teacher is not coming down on you or your boy, she is simply trying to figure out if there is anything more going on than just your boy being "quirky". Having temper tantrums are normal at this age, but he might need that extra bit of therapy to make that transition easier from one thing to the next.
For your own peace of mind, go and have him evaluated. They might tell you there is nothing wrong with him, and you can plan your life as you were.. or they might tell you different. Just be prepared. Prevention is better than the cure. The earlier you can catch these little behavioral problems as they call it, the easier it will be to deal with them.
Good luck.