In the last month, my boyfriend didn't take the initiative to contact me, send messages or make phone calls. I sent a message and he replied, saying that he missed me or something.

That's right. Anyway, don't make a wild guess. If he doesn't contact you, you should contact him actively. If you don't contact him, you may actually feel the same as you, "Why didn't she contact me?" So, no matter what, you can contact him first, ask him why he hasn't contacted you recently, doesn't miss you, is something unhappy, and you can give him a guide. If you think it's because of something, you two can work it out little by little. If he is still cold and unable to communicate with each other, it may be his problem. But long-distance relationships are prone to problems, so you must take the initiative to solve them. You know, sometimes the other person thinks the same as you, that is, you all think so but don't take the initiative to take a step. In short, don't judge anything blindly, let alone think. Communication is the right way.