You should definitely talk to them about it. If you guys share everything, make this a thing. Say outright that you feel that there's something weird, ask what's going on. If they don't start a conversation with you about it, be gentle, but let them know how it's making you feel, and that if they have a problem, you want to help with it, or be there with it. If they won't talk to you about it, but it's a thing they're going through not a problem with you, then just wait it out while they deal with it, and keep being the good friend you are while they go through whatever it is. If it's a simpler problem, like they've started a romance, or someone or something is coming between the two of you, you need to hash that out, and make a decision about how to move forward from there with what may be a new dynamic. It's always rough when relationships heel-turn, but talking often helps and rarely makes it worse.