Xperia Pro/Neo/V/L :: Part Of The Content On SD Card Missing

I own a Sony Ericsson Xperia Neo, and I just bought it less than a month ago. A peculiar thing happened to me today- my phone (or my SD card) simply decided to "forget" all the stuff that I had saved on it (on the SD card), more precisely, copied from another phone and from the laptop. It erased ONLY the things I saved there (my music and pictures from my laptop and my previous phone, and probably other stuff that I don't even remember I had on it). All the folders that the phone itself created on my SD card (camera pictures, Bluetooth, downloads, folders linked to apps like other camera apps and so on) stayed untouched. I first noticed that something strange was going on when I connected my Xperia Neo to my laptop in order to copy some music to it. When I opened the folder I created on the card, only some of the data was there, not all of it. I disconnected the phone, and unmounted the SD card, took it out, and then put it back in, I think I did this more than once. When I put it back in, the thing mentioned in the beginning happened- all the stuff that I put there was gone. I checked the card once more by putting it in my laptop, and there it was, half-empty. Does anybody know what the problem could be? I'm confused because of the arbitrariness (I don't know what else should I call it) of the items removed. I thought I accidentally touched the "Erase SD card" option, instead of Unmount SD card, but then it would erase ALL the content on it, wouldn't it? I just wanted to check if anybody had a similar problem and could just explain to me what happened, I am aware that I will not get my music and pictures and other files back, I simply cannot understand how this is possible. No other problems with the phone, except for my display turning on/off on its own, but I thing I got that fixed with Automatic Task Killer app.