Changed due dates?

Hello,

When i first found out i was pregnant doctor gave me a due date of 8th Feb. I went for a scan at what i thought was 7weeks. They said i was 5 weeks amd gave me a new date of 18th Feb. I just had my 12week scan and was told i was 13 weeks 5 days with a new date of 11feb. No ones told me what I should go by? Is it normal for dates to change so much? TIA