I agree that the problem is likely to be the result of that Fix-a-Flat junk.
While I would be tempted to use it in an emergency (remote area, no cell phone service, spare tire also flat), I don't think that I would consider the OP's situation to have been an emergency. However, since the product's packaging does not warn purchasers that this junk will probably unbalance a tire, the OP is not to blame for using this stuff.
As was said, take the car to a tire shop in order to have the tire dismounted, cleaned out, and fixed properly with a patch on the inside of the tire, combined with a plug to seal the puncture from the outside. Just don't be surprised if the tire shop charges a high rate to clean that goop out of the tire. Mechanics and tire technicians hate dealing with that stuff!