As a previous loss mama I won't even tell you not to worry, because I know that's pretty much impossible!
From my own experience, my breast pain is ALL over the place! It was really THERE for like the first week, and then GONE completely, and then it started coming every few days in varying degrees. Some days they are pretty painful for a few hours and then I feel nothing at all! It's just really random!
I was worried a little by the craziness of my symptoms (MY MS even went totally MIA for a bit, which scared the hell out of me!!), but we saw the bean yesterday alive and squirming, so I'm feeling much better.
With my first pregnancy my breasts were terribly painful, burning, tingly ALL the time right through the first several months, so to have such mild and sometimes non-existent symptoms is weird and definitely can be worrisome! But I think since we've BTDT and already made milk and nursed, our boobs just don't have to go through so many changes. And especially since you are still nursing now, your boobs are probably like "yeah, we've got this! no biggie!" after the initial hormonal increase.
Good luck mama! I hope everything goes beautifully for you!