Opportunity start date is getting changed

Hi Team,

I have created some opportunities in the month of march this year. Now when I check these opprtunities, for few of the opportunities the start date is changed to future date, for example, if my start date was 02.03.2013 , now it got changed to 31.12.2013. In the change history , I could see my user against the new date. But I have not made any changes to the document.

Could you please help me regarding this.