@teroobaau-- When did you OPT extension start-- Nov 20th or Feb 16th?

If your OPT extension started on Nov 20th and you had used 45 days of unemployment before that,
You had 75 days(120-45) of unemployment left. So, Nov 20th to Feb 5th(approximately). So you have to have paychecks starting Feb 5th(approximately).  

If your OPT extension started on Feb 16th and you had used 45 days of unemployment before that,
You had 75 days(120-45) of unemployment left. So, Feb 16th to May 2nd(approximately). So you have to have paychecks starting May 2nd(approximately).  


P.S. When you say you had 45 days of unemployment before the extension, you mean you weren't working at all right? On regular OPT even unpaid work can count as 'working'