Someday ago I got one mail from Hosting Service provide stating that
"We have received a request that the name servers be changed for the following domain name(s):www.mydomain.com (original domain name changed)”
I did not request anything from them; I can open my hosting account and can make necessary changes. my domain is 3 months old.
Is someone trying to still my domain or anything else? What should I do? Can anyone explain me what is going on?

Yes it could be the security issue or may be your password is weak. Only you (your registered email) has authority to change the DNS of your domain. In such case domain authority can be changed.

If you didn't make the change, someone with access to your account certainly did.

You should change your password and also check your contact info (administrative, tech, etc.) for your domain to make sure it has not been changed.

I'd definitely change the password on that account, and verify the contact info. I'd also check with your provider to ascertain who made that request.

Agree with everything posted here so far - if you didn't request it, someone has your password (or your host/registrar did something they shouldn't have done).

Change your passwords, see who requested the change, and also be sure to LOCK your domain so that nobody can TAKE the domain from you. Will also want to change the access password on the email account associated with the domain name for good measure.

VERY good suggestion.

Does it show last logged in IP in your domain control area?

It seems some one hacked your account, try to find the virus/trojan on your personal computer. Most of the trojan program is designed to steal user password so make sure that you have installed all security updates in your local system (update all antivirus and try also some softwares like malwarebytes and anti-malware). Also, you must have a strong password with special characters, letters, and numbers (at least 12 characters). Only temporarily store this password, do NOT store in browser.

I would call your Registrar and make sure of the change and let them know you did not authorize any change