Its probably due to a narrow aperture. On professional grade lenses, you can usually stop down a bit and still maintain a rounded aperture, however on cheaper lenses, or on all lenses at very narrow apertures, the opening becomes polygonal. That causes the diffraction of light as it passes through the aperture to produce a star pattern (the exact nature of which depends on the number of sides/points in the polygonal opening.) Use a stronger ND filter or and/or a lower ISO setting to reduce the effect.
Given that you are using the kit 18-55mm lens, its unlikely the diaphragm uses rounded blades. Star patterns will start to exhibit pretty early, and get stronger the more you stop down. If you have the cash, you could try getting a higher quality lens. Most of the L-series lenses have rounded aperture blades, which eliminates the effect at the wider stops, and mitigates it in the middle stops (not much you can do once you get down to f/11, f/16, f/22 though.)