We were driving down the freeway and all the sudden the check engine light started to flash the car still seem to be running fine but when we came to a stop you could feel the engine shaking we also noticed a strong smell of sulfur what could the problem be

 

When the light flashes like that it is serious. Read your codes and find the problem ASAP. It could ruin your cat among other things.
Could be a crossfire, one or more bad coil packs and other problems. Your OBD2 code reader will tell you what codes pop up.