'97 Plymouth Voyager. Burning smell.

I am having problems with this awful burning smell which I noticed about a month ago or so. I notice it when im in park and the windows are open I can smell it if the wind hits it just right or when I shut off the engine and get out of the van I can smell it. A little over a week ago I had my water pump, timing belt and drive belt replaced. Then last week I had the oil changed and the fuel injector cleaned and I couldnt smell it anymore and now im beginning to noticed this awful smell of burned whatever it is again. What could this be? Help!