I could have written your post word for word except mine lives 1 hour away and we don't talk as much on the phone.
I personally think it's a red flag.
It takes literally 2 seconds to send a text.
My guy and I have been texting for 2 and a half months, but have only seen each other 7 times. In the beginning he was texting like crazy. Several times a day, random stuff, long texts too, not just one word responses. I too valued these text messages because I knew we couldn't see each other that often.
Then suddenly about two weeks ago (the 2 month mark) they drastically changed. Sometimes I would send a text and he wouldn't reply. Hours would go by and nothing. Then if he did text, suddenly it was really short and generic.
I know he is glued to his phone. It is always on him. So when I don't get a reply, I know he is ignoring me. I don't keep send texts. If he backs off, I back off...that's just how I am. I'm not going to chase anyone...but to tell you the truth it does drive me nuts.
I also noticed when we are together, (his phone is on silence...kind of a trigger for me) he is constantly being contacted by someone. He is always looking at his phone and texting. He claims it's his kids. I don't know.
If you guys are talking like normal, then that is a good sign. But I'm with you, when I see a pattern shift, I think something is up.
Good luck! I hope it's nothing.