Check the seal on your back window, I know that sounds weetarded, but if your slider window (assuming you have one) got wowed out somehow, it can leak from the black window flashing. (I am also going to guess flashing isn't the ideal term here for the black weatherstripping stuff around the outside edge of the window)
if any pine needles, leaves or debris has worked its way into that stuff surrounding the opening of your window and created the SLIGHTEST opening... well you guessed correctly, in the abundant rain we've had as of lately, a little seepage could start, and it will run down that back wall under the window and collect on the floor, at the path of least resistance.