At least you weren't hurt, though I hope you got checked out by a doctor just in case.
I too once was rear ended by a semi. I know instinctively you want to slam on the brakes when you get hit, but boy was it a mistake for me. The semi bumped me and I went forward a bit then the truck hit me again and kept pushing me until I realized I had my foot on the brake.
The sound I remember most was the sound of rubber just scraping the asphalt... and looking in my rear view and seeing that big arse grill! I couldn't sleep that night