As a former preschool teacher and parent, I tend to stand with the "believe half of what you hear" sort of thing. Meaning, that you are doing exactly the right thing-- listening, believing,and also wondering what your son's role is in this situation.
Good for you.
I'd make an appointment to talk to the teacher (conversations are best when teachers aren't greeting the other families or 'on the fly', so both parties can concentrate) and ask what she's seeing. I'd also ask if your son could be moved in the classroom /floor time to see if the dynamic changes. You'll know if your son moves and starts having the same problem near a different group of children that maybe there's something in his interactions contributing to the situation. Or the teacher may notice that other child picking on the person who took his space. As a classroom volunteer during my son's time at public school, there were some kids who would always find others to pick on. I know one child who used to torment those around her by getting in their face and annoying them until they would yell at her or push her away, and there were a few really nice boys who she would target regularly. Fortunately, the teacher was on to her at a certain point, but she'd just be flat out mean.
On the other hand, my own child had his own challenges with not respecting personal space. This did cause conflict, including being hit, and we had to work with him on respecting that 'personal bubble'. I found social stories to augment this and will still, sometimes, say "personal space please" when things feel too close for me. I'm an adult and have mastered 'using my words', as they say, but young children do not have that finesse and will often get angry, hit, push, etc because their space isn't respected. So, great that you are keeping an open mind.
Again, schedule a phone conference with her, or some time to meet before/after school and make sure to know-- while these things feel 'big' to us parents, they are likely small potatoes to the teachers who have seen it all before, over and over. One of the boys who really rejected my son when he couldn't regulate his 'space'--- now they are buddies. Your son will learn a lot of things about others in kindergarten! So, schedule the call and remember, too, not to make a Big Deal out of this for your kiddo. Don't question him or ask him "how was so and so treating you today?"... if it's important, he'll tell you, as he has already done. You don't want to encourage him to focus on that... instead, also, you can ask him "so, what could you do if that happens again?" and focus on reasonable options. Good luck!