Definitely replace the helmet. Remember, you have plastic covering the outside, so cracks in the foam could be hiding underneath the plastic caused by the impact. If you purchased your helmet within the last year there could be a crash replacement warranty, depending on brand. I know Garneau offers 1 yr crash replacement, all handled through LBS. Just go in, give it back, get a new one of equal value, the shop should handle the rest. I think bell and Giro also have something for crash replacement, not positive though.

Definitely get a new helmet though if you think you hit your head.