Could be that there's no dedicated ground, only a hot and a neutral wire. Only the newer homes here are wired with three wires. In fact, you can't even buy romex with three wires at Home Depot...they don't sell it. So even if it's a three-prong outlet, you may want to pull it out and see if it's wired with a ground wire.

With all concrete walls, running a ground wire all the way back to the electrical panel and then putting in the proper ground there might be really complicated. Then again, since you own the house, it might be worth the effort. I'd consult an electrician. The work here is so cheap, it'd definitely be worth it.

The other possibility is that the polarity is reversed by connecting the wires to the wrong terminals on the outlet (hot to neutral and neutral to hot).