Hi Finally i got the solution.
Here i am using getResponse = (HttpWebResponse)getRequest.GetResponse();
The problem is we can use only one HttpWebResponse at a time. In my case i am using the same object in two times without any disposing and closing. That's make me the error.
So I updated my code like this.
byteArray = Encoding.ASCII.GetBytes(postData);
getRequest.ContentLength = byteArray.Length;
newStream = getRequest.GetRequestStream(); //open connection
newStream.Write(byteArray, 0, byteArray.Length); // Send the data.
getResponse = (HttpWebResponse)getRequest.GetResponse();
getresponse.Close();
This solved my error. Thanks