Most Helmets are designed for a single impact, could be time to replace or have the mfg inspect. I did a similar heel side smack back in 1999 with a hard shell Leedom lid. I could not detect any damage but Leedom said the impact was so hard it fractured the styrofoam at the point of impact and the reprocussion fractured the front. The cost to rebuild was $30 + shipping.
When to Replace a Helmet
A helmet will generally last 2-3 years under ideal conditions. However, a hard fall or other impact can render your helmet useless. Frequently inspect your helmet for cracks, dents, imprints, etc. Don't take risks. When in doubt, replace your helmet to save your noggin.