Can't work out what he's thinking?

So lately I [18f] was doing really well after being dumped by the guy [19M] I was seeing for 6 months and really liked, still thought about him but not nearly as much and was starting to feel normal again

Then the other day he got back in touch and I was fine at first, but after some messaging, he hasn't replied in a day which he's not the type to do. The first few hours didn't bother me and I was fine and didn't really think about it. But now it's been over 24 hours I'm wondering why? It's him that started the conversation and he's been online, not all last night since my last message (12+ hours he wasn't online) but he was this morning - the chat isn't anything heavy either just general "how's it going stuff".

I can't work out why he's gotten in touch either as I think him wanting to get back together is highly unlikely. He said he really does want to be friends but I didn't actually think he'd get back in touch especially after a month NC. I'm just really confused and don't understand why he's not replying when he started the conversation. I also really hope he isn't seeing someone else because that's a shame for the other girl. Messaging the last girl you were seeing is shady even if it is in a friendly way.

I'm just annoyed because I feel if we had a chat and it just ended naturally I would be fine and would let it go. It's the fact he's not even opened my message and has been online. I truly feel a short conversation would've been fine but its the fact he's started a conversation and not finished it. Surely if he just wanted a catch-up he would just get it done and over with? I'm just annoyed as I said because of how much I was getting over it and now I'm thinking about it again. Any insights?

tl;dr ex got in touch after a month of NC then left the chat for over a day without even reading my message. Was doing well but now can't stop thinking about it.