It sounds like it could also be an unfortunate case where they may just simply not feel as close to you as you thought they did or as close as you feel towards them. It hurts, it really does and I've had this happen to me many many times. It's happened so many times that I actually expect people to do this to me and don't really get close to others now.

If they claim they're too busy, that's their way of just trying to avoid hanging out and talking to you. Yes everyone is busy, but in the age of technology at your fingertips, there's absolutely no excuse why they can't send a brief message. People even use their phones at work during breaks or brief moments of downtime. No one is busy 24/7 so they should be able to text or call you back. It sounds like they are making the conscious choice to slowly end the friendship. I hope you are able to make new friends and I know that can be hard. I'm always here if you want to talk or message me.