I really don't understand why he didn't return my text messages …

I haven't answered the message. I may have missed it. After all, I had to send n messages that day.

If he doesn't reply to more than three short messages, it can explain the change of his attitude.

Don't make wild guesses, you will confuse the fact (a short message didn't return) with speculation (he doesn't value you).

Face it frankly and pretend that nothing happened.