At Wk 11 My Placenta Was 8 Cm Away From

At wk 11 my placenta was 8 cm away from my cervix. Also at week 24 it was more than 7cm (as written in my ultrasound paper). It changed to 2.4 cm at 29 wk+6 days
My doctor said maybe that last ultrasound might show the wrong dimensions. Anyway, the doctor said no running and sex. But it's OK to do your regular chores and walking is also fine.
Can you advise why this has occurred? Does it mean by my 36 wk ultrasound the placenta may cover my cervix makes it difficult to deliver vaginally or maybe the baby should come earlier in case of bleeding? I'm not blessing at all right now. Thank you