Bear with me, this is going to be long!!!

So E had an incident at school the other day where she was pushed over by a little guy in her class and hit the ground. She is fine (physically), he was corrected and they consolled her . I'm not at all upset by the incident, kids will be kids and crap happens. She was pretty emotional about it all ("Mommy, hurts my back on the floor!"). Her teachers tried to help her understand that she can and should advocate for herself and tell her friends 'no' if they are bothering her.
She hasn't been herself since. She didn't nap that day at school, had a terrible nights sleep that night (lots of crying and wakeups, none of which usually happen) and hasn't wanted to go back to school since. We're going to give her a couple more days to get back on track before we chat with her teachers about a strategy to address her ongoing issues.
On our end, we're trying to find a way we can help give her confidence that saying No to friends means you will be left alone and you can feel safe.... but when she says no to us we correct her pretty sternly. Think "Time to clean up your toys." answered by "No." brings on a "You don't say no to mommy and daddy. You listen to us and do as we say." type conversation.
So, with all that in mind, how do I make a 3yo understand that sometimes she can say no and it matters, but other times saying no is not acceptable? Any insight!?