To me, this sounds like channeling. When you brew espresso the machine pushes water through a puck of compressed coffee grounds. Ideally, the grounds are distributed equally throughout the puck so that the water passes through the puck evenly. In reality, there will always be small inconsistencies. These inconsistencies allow water to move through slightly faster, possibly moving some of the inconsistent grounds along the way. When too many grounds are moved so that there is an opening from the top to the bottom of the puck, you've got a channel.
The channel provides a path of low resistance through the puck, so a lot of the water will pass through there rather than through the compressed parts of the puck. This extra water flow through the channel will widen it even more.
Water passing through the channel comes into contact with only a small portion of the grounds in the puck. As such, the grounds around the channel are overextracted while the water coming out is lightly colored and overly bitter. It's lightly colored because it has less contact with the grounds. Nevertheless, it will be bitter because the grounds in the channel are already fully extracted.
To confirm that your puck has actually suffered from channeling, simply look at the top of the puck after brewing. If the channel is big enough, you will see some holes in the puck (while it's still in the portafilter).
If you have a naked portafilter (one where you can see the bottom of the filter basket) you will be able to see channeling during the brew process. An even extraction will be a gentle singular stream whereas channeling will look like thin high pressure streams. Because a video says more than a wall of text, I'd recommend watching this video by Whole Latte Love on YouTube. Even if you don't have a naked portafilter, it will be interesting to see how espresso exits the portafilter. Note that the shots in the video are all pretty good with some minor channeling. If the tamp is really bad (not shown in the video), I would expect there to be many more of those tiny jets which turn lighter in color fairly quickly.