Check undo / rollback usage
select s.username, rn.name, rs.curext
,rs.curblk, t.used_ublk, t.used_urec
from v$transaction t
,v$session s
,v$rollname rn
,v$rollstat rs
where t.addr = s.taddr
and t.xidusn = rn.usn
and rn.usn = rs.usn