Spore replaced my whole game- help!

I had lost track of Spore on my computer for about 4 months (it seems to uninstall itself occassionally) when I finally found a backup version somewhere I definitely did NOT put it. When I opened the game, I found that all of my planets were still there, to my astonishment. I popped into my Space stage game (on planet Medhir, to simplify things) and everything seemed to be in working order, then I played another planet on Civilization for a while.

I quit the game, and when I returned a couple hours later, everything had been turned upside-down. Everything on Medhir had been completely replaced; that is to say, all of my creations had been swapped out for random stuff by other users. My progress had been preserved, but the species I was playing as was no longer mine, my ship wasn't mine, and none of my houses or vehicles were mine. The creature files for the species I evolved on Medhir are still in the Sporeopedia, though a couple iterations are missing, and some of my architecture is there as well. They've been difficult to track down, so it's unclear how many files are actually in the Sporeopedia.

What the heck happened? Why did it replace everything, especially when some of my own creations are still easily accessible (I have no idea what could have happened to the others in the space of two hours when I wasn't even on the computer). How do I plug in my own creations again?