Yes, it happens to me as well.. in the morning, i didn't feel tight but in the evening, I thought I tigthen too much but it could be like you said, expended later in the day.I got them in Mothercare at $59. Quite expensive but the material is soft. My chest is expanding too fast! It was slightly loose when i tightened the first buckle and now i am on my last and feeling the restriction now (in ten days). My body change tremendously starting from 18th weeks. I also bought 2 wireless bra from M&S, quite cheap and still works.
I noticed that in the morning, I could buckle tighter and when it comes to afternoon, my chest expand quite alot. I got to loosen the buckle. Any one has this problem?