why is toad connecting with lower case schema name when I typed in upper case?
Actually, Toad uppercases the username, unless you surround it in double-quotes.
Schema (user) names are not generally case sensitive, TOAD is not connecting in either upper case or lower case, it is just connecting to the schema. How it displays the connection string has nothing to do with the actual connection.
As with other databse objects such as table, views etc., they are normally stored in upper case, but it makes absolutely no difference whether you specify the schema name as MYUSER or as myuser, or as MyUser, or any other combination of case you like.
BUT - there is an exception if you enclose the schema name in double quotes when creating it. You can then create usernames in mixed case, although it can then be tricky to connect to them (you have to enclose the username in quotes again in order to NOT have it converted automatically to upper case).
Actually, Toad does uppercase the username before the connection is made. We do this so that we have a case-insensitive login history. When Toad connects, it uses the uppercase, unless you surround it in double quotes. In addition to Double-quoted user names, sometimes case sensitivity is needed for certain types of external or global authentication.