Service Rules and Previous Job

Sir i am ssc/upsc aspirant. when i was in btech (graduation) final year at that time due to financial crisis i worked in call center(private company) for 9 months. though my college was regular i manged as job was night shift. Further i resigned and got relieving letter from that company. While filling application form form for bank PO i did not mention about that job now that i am selected and i about to join bank PO i m afraid that mentioning at this stage can jeopardize my bank PO job. should i keep hiding. what should i do pls help.
Also i want to know if i should mention about that call center job with regular graduation while applying for ssc/upsc or hide it . is it possible that ssc/upsc will cancel my candidature because i was doing private job while pursuing regular graduation. please help