When stopped at a light, it felt like someone hit me from the rear. What could that be.

Hello, honestly, as someone who has been in automotive service for nearly 30 years, I can tell you: I can't even conceive of what you're experiencing. You're stopped and it feels like someone hits you? I'm so sorry, I can't even theorize what this could be. Your best bet is to show someone what is happening when it happens. If it's not happening frequently enough to show someone, then maybe it's not a concern. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops