I can't enter the servers unless i only have letters in my name?

I've been playing on this server for a long time and I've never had any problems with entering the servers with a smiley in my name.
I know this isn't any major problem but i just want to know why. I don't have any problems joining any other server with a smiley in my name.
When i enter the server it says: "Please, change your name"

Perhaps this is just my problem or has already been told why it happens?
(Or someone just thought i shouldn't use that stupid name...)

Does anyone else have the same problem?

Generally this happens when your name contains invalid unicode characters. Most BMP and SMP characters should be fine as I see players joining with crazy looking unicode names all the time. I'd check that your name wasn't too long and isn't being truncated.

I tried with the same name as i had the day before and now it keeps telling me to change my name.

Blind guess: You're using the name someone else is using.
im late i know

Not really, i tried with a lot of other names too.

As I mentioned previously it's all caused by having invalid unicode characters in your name.