I assume this is your first pregnancy? Some good advice I got from my GP when I was pg the first time - you need to learn to let go and ride with the waves a bit, it will be like this from now on! Even if there is a reason your boobs aren't sore (which I'm sure there isn't) I expect it is just natural fluctuations in their expansion - there isn't much you can do about it!
I am pregnant with my third and my boobs are not demonstrating any soreness whatsoever as they have done it all before! But this time I have all day nausea. It is different every time.