When your coffee is running poorly and tasting strange it is usually one of two culprits – the grind / texture of the coffee or the dose / amount of ground coffee in your handle. These two factors work in conjunction with each other. If the coffee is too fine it is like water moving through sand.
The tiny bits of grind pack neatly, allowing very small gaps for the water from the machine to travel through, so it seeps through, over extracting the coffee and creating a dark, bitter, oily shot. The same effect will appear if the coffee is packed in too tightly with a heavy tamp forcing too much coffee into the basket. If the coffee is too coarse it is like water through stones or gravel.
Larger pieces of ground coffee pack in unevenly, creating gaps for the water to pass through easily, so it rushes past, under-extracting and picking up tangy, sour flavours. The shot can taste watery and sour when the dose is too low – if there is now enough coffee the water will flow through the handle just as easily.