There are a lot of reasons that would cause the vaginal walls to thicken. One is estrogen, which dominates during the first half of the menstrual cycle, causes increased mitotic activity, cell growth, thickening of the endometrial lining, and up-regulates cell sensitivity to itself.
Another is, in adenomyosis the lining of the uterus infiltrates the wall of the uterus, causing the wall to thicken and the uterus to enlarge.
But I think the best way for you to find out, is to ask your physician, even if you just call the office and ask. Because I am sure that you may get many explanations but if that isn't your experience, than it won't be your diagnosis either! I would ask my physician to explain.
Also my experience with this is that mine was associated with cancer...so there are so many reasons why this could be happening, you would really want to know the one that pertains to you.
"Estrus (is) the period in the sexual cycle of female mammals, except the higher primates, during which they are in heat?i.e., ready to accept a male and to mate. One or more periods of estrus may occur during the breeding season of a species. Prior to ovulation the endometrium (uterine lining) thickens, in preparation for holding the fertilized ova. As the proliferation of uterine tissue reaches its peak, receptivity is highest?this is the estrous period."