Thank you Jurij,
May I ask you to explain this paragraph please:
'If your TEMP tablespace is of type PERMAMNENT and your PCTINCREASE on the tablespace is set to something different than 0 (as it never should be!), SMON will coalesce it all the time and you will see different amount of used space in it. When there will be no sorting on the system for a longer period of time you'll see no used space in temp tablespace at all. '
I mistakenly though coalescing is a good thing even for a temp tablespace because it'll combaine free extents into large chunks and it is a good thing.
THANK YOU.