just have a look at the segments in your TEMP tablespace, if owner is SYS, then these are temporary segments, and it's a perfectly normal behaviour (Oracle fills the tablespace, and then just reuses unused segments when needed), if not then you should alter your tablespace and make it temporary (alter tablespace TEMP temporary) to prevent people from creating objects in this tablespace.
I do not understand why this happened just after you added your datafile, maybe you just didn't see it before ... well anyway, you should free unused space from your temp tablespace by using the now famous workaround :

alter tablespace TEMP default storage (pctincrease 1);
alter tablespace TEMP default storage (pctincrease 0);
alter tablespace TEMP coalesce;