What to do when situationship (19m)says he’s sad but won’t talk about it (20f)?

My situationship of 4 months randomly texted me that he was sad and was going to go to bed early and that he didn’t want to talk about it.

Backstory: we’ve dated before for about 2 and a half years and then split. Now it’s turned into more of a situationship.

I responded and asked if it was sadness in general or something specific because obviously I was worried. He said “specific but I DONT want to talk about it. I’m fine.” I was trying to be respectful but at this point I’m really worried. So I called him and I just asked him why he bothered to tell me he was sad but won’t tell me why because it just felt like he dumped this on me and refused to give me any clarity. For clarification there have been other girls in the picture and lots of lies on his end so my head went to “what if it has to do with another girl.” So I said nothing terrible happened right? and he says no I’m fine. Then I said it doesn’t have something to do with another girl does it? And he says no and is immediately annoyed so I just said okay and goodbye and the call ended. The whole situation seems off to me and I don’t feel like I was being to pushy about it I just wanted reassurance but at the same time I feel like something odd might be going on and that he’s going to say I was being disrespectful. Am I right to feel this way or am I reading into it and not being respectful?

TL;DR situationship said he was sad and refused to tell me why. This has never happened before. I asked follow up questions and he got annoyed. Am I in the wrong or being disrespectful for being suspicious and anxious?