Measuring 1 day behind

This may sound ridiculous to some idk but that’s why I am asking ..

According to the last day of my period I should be 9 weeks and and 4 days.

Last time I went to the doctor I was measuring 1 day ahead which would mean today I would really be 9 weeks and 5 days

Today I went to the doctor and I was only measuring 9 weeks and 4 days. I know it’s only a day but where my head goes is something wrong? The doctor said I’m right on schedule I’m just confused why it’s behind a day now.

Does anyone have any experience similar to this?