1 year post-op. Why are my teeth moving even after wearing nightly Invisalign retainers?

I completed invaslign treatment, almost 1 year ago. I was given a set of 10 passive aligners as lifetime retainers. During the year, I have worn 3 of them so far (switching every few months due to wear, aside from the 3rd set, which I wore for 5 months -- the longest time) I have now switched into the 4th one and ALL My top teeth HURT and are moving. Therefore, my teeth have moved despite wearing my passive aligners as retainers nightly! Why is this happening? I am really worried.