If you don't specify the tablespace for lob segment then oracle will create it in the same tablespace where the table is created.

You can find the tablespace of the lob segment by querying USER/DBA_LOBS and USER/DBA_SEGMENTS;

SELECT t.table_name, t.column_name, s.tablespace_name AS lob_tablespace
FROM USER_SEGMENTS s, USER_LOBS l, USER_TAB_COLUMNS t
WHERE s.segment_name = l.segment_name
AND l.table_name = t.table_name
AND l.column_name = t.column_name
AND t.table_name = 'MY_TABLE'
AND t.column_name = 'MY_CLOB_COLUMN';