I'm a perfect candidate to answer your question! Movable is a GOOD thing! If it weren't movable, then I'd be more worried. I've struggled with the same issue for years. I have a very large balloon like lump on my left breast by my armpit. It will swell up sometimes and hurt. It has always scared me but I never had insurance to find out what caused it.
Would you say your breasts feel slightly lumpy when you press into them (not look lumpy, just feel that way)? Do they swell much more when you are about to start your period? Does wearing bras make it worse sometimes?
One day, my fiancÚ laid on my breast while we were watching tv. He accidently plopped his head down pretty hard onto the balloon like swelling (which I thought was my lymph node). The next day that one breast was so swollen, red, inflamed, and painful I couldn't stand it. My friend scared me by telling me it could be this rare form of cancer called inflammatory breast cancer (tends to swell in just one breast and become inflamed).
I found a program that allowed me to get a free mammogram through a local university. When I got there, because I was so young, they decided to just do an ultrasound first. I watched as they checked it all out. So that balloon like thing by my armpit ended up being a very large water filled cyst! Apparently my breasts were riddled with them. I had hundreds of black circles on my ultrasound. The doctor said that if they were grey on the screen, that's when we worry. Apparently it's a fairly common thing for women. When my fiancÚ "head butted" me it had burst that large cyst on the side, hence the swelling and pain.
If that's what you have, there sadly isn't much you can do. The doc said that there are procedures to remove them but they generally always come back in the same spots. I've found that not wearing my bra as much helped out tremendously!
So anyhoo, I think you may have the same issue. However, if you can get it checked out by a doctor you should, for your peace of mind. Hope this helps :)