So, the diameter of the uterus would change for different reasons but I have to ask why you are having it measured month to month. ? I think the only time they measured my uterus was during pregnancy to see if it was growing. Are you able to feel it enlarging or bulging? Things that can cause it are pregnancy, obviously. But also fibroids? Ever been diagnosed with those? Usually you have other symptoms if you have fibroids like bleeding. There is something called adenomyosis which is when the endometrium (uterine lining) grows into the uterine wall tied to estrogen production. This also normally has heavy bleeding associated with it and cramping. And I guess cancer could be a concern. (but let's not go there until we have to).
Anyway, you have been very anxious about things for quite some time. You are not pregnant from what I recall from your other posts. Maybe talk to a doctor about anxiety. good luck