Broken bottom bracket? Pedals are pretty loose
Here's a quick test. Grab the crank arm that you say is moving and wiggle it. Does the arm on the other side move? If so, you've probably got a bottom bracket problem. The answer then depends on whether you have a sealed or loose bearing botton bracket. As mentioned previously, a sealed shouldn't do this after 2600 miles. (Neither should a loose bearing, but it is more likely).

If the other arm is not moving, then the one wiggling is probably coming off of the bottom bracket spindle (the axle). The spindle may be rounded off, or the crank arm may be rounded off. The only way to really tell is to take it off and see. This probably means that it wasn't installed securely in the first place.