Can I be sure it's a rebound?!?

So my ex of two years and I have been apart for a month now I’m on day 20 of NC and this whole time she has been getting really friendly with a guy who I did consider my best friend here at school. Now I’m hearing that they might be a thing and people had told me around the end of the relationship that they thought there was something going on between them. So now I’m scared that maybe she left me because she liked him. So does that still mean rebound?