Due date changed

Hey guys,

So I went for a scan at what I thought was 7 weeks, but I measured 6 weeks and 2 days with a heart beat of 116 but I thought that would be the case as my cycles are long and I ovulate late.

I went again today because of some concerns and I thought I was 8 weeks and 5 days but measured 8 weeks exactly but the heart beat was strong at 170.

Is it normal for your due date etc to keep changing like this is your first trimester? Has this happened to anyone else?