Originally Posted by Weasel

Flashing engine light means only one thing... misfire. If you just had spark plugs changed and it is now misfiring I'd say the shop needs a call. You will get the diagnosis done for free at the very least. It is most likely either a cracked porcelain on a spark plug (which requires careful extraction to keep pieces from getting in the cylinder) or a coil is going out. If it is a coil that can be confirmed by swapping it to another cylinder and seeing if the fault follows it.