Ex nice to me then acts uninterested? Help

Me and my ex bf were together for 5 years, he broke things off with me in June for ridiculous reasons. We met up a few times since then, and ended up arguing. We didn't speak for a month after that. Then, this week, he started to text me saying he had wrote a song including me, and asking how uni was etc. We then saw each other randomly in public and he was being nice to me.

So, I thought I would ask him if he wanted to meet at the weekend as we were getting on well. I called him to ask, then he says 'no I am seeing someone' and that he isn't interested in me.

This really hurts. I don't understand why he was nice to me all week, then when I asked to meet, he goes cold on me. Can someone help?

I didn't want to get back together, I just wanted to meet to chat as we got on well but now he is acting uninterested in me, as if he doesn't care.