Did you ask your doctor? I'm curious as well. I know we "shrink" as we age (in height, not in weight---unfortunately
). However, 31 seems way too young for this to happen. I'm wondering if somehow you were measured incorrectly either the last time or this time. Maybe you should have someone measure you at home just to check.
I know that when I was in high school, I was measured as 5 ft 4. Then, years later my husband measured me as 5 ft. 3.