If a plug change "fixed" it temporarilly and it went sour in a couple days, I would pull the plugs again and take a look at them. http://www.gnttype.org/techarea/engine/plugs.html will show you what to look for. Very likely there is another sensor malfunction causing a too rich/too lean situation (an oxygen sensor gone bad is a common fuel mixture problem cause but by no means the only one), or perhaps oil blow by indicating worn piston rings (among other causes) or..... Basically there are many causes from a corroded wire needing to be cleaned to a blown motor. Putting an eyeball on the spark plugs should at least give you some useful ammo to decide to trust or not trust whomever gives you their "professional" diagnosis...since you just changed them and it did have a positive effect for a short while. Hopefully it's just running real rich due to a faulty $60 sensor and the plug won't fire any more...and you find a reputable mechanic that can fix it for about $100. It's also possible a ripoff mechanic could say it's a $2,500 problem or a good solid pro could tell you and confirm it's a big problem. No way to tell for certain from the info you gave...but you can at least have an idea of the cause by comparing the plug pics to the link I posted. Best wishes!

double check the work just in case something came loose by chance. also it is entirely possible that the plugs you bought were junk. i have had this happen before.