Sorry to hear about the bitter taste getting worse after the filtration, it always is disappointing when something we've put a lot of time/money/effort into doesn't go well. Luckily, we can usually learn from our mistakes and avoid them for the next batch
So I suspect it was in fact a contaminant. Are your cleaning and sanitizing procedures sound? To answer the question why the filtration may have made it worse: filtration takes out a lot of yeast and other crud that contribute to off flavors. Once those are gone, you are left with a much cleaner taste. However, this also means that any flavors remaining -good or bad - are now more apparent with those other particulate contributing entities gone. Think about it like static noise in a stereo. If the static goes away, you can hear what you are trying to listen to better. Only with mead, everything will be more apparent. If your mead was already pretty clear, the change probably wouldn't have been too apparent, but if it was a hazy mead that may have made the difference. Thats why I like to wait at least 6 months now before I do any back sweetening or adding spices. I don't want to inadvertently over do it by competing with "static noise" that will eventually go away, leaving the backsweetening or spices too strong later on down the road.
I happened to use my buono filter today on a dry raspberry melomel and a semi-sweet goldenrod trad. In my post below I mentioned that I tasted a soapy/papery taste initially (the exact flavor that you smell from the wet filtration pads, so I know thats where its coming from). As a test, I continued to taste a bit of the filtered mead every once in a while (from the drip tray) and noticed that this flavor actually remains quite strong for a long time, and its quite undrinkable. Luckily, I could not detect it in the final product when I tested it from the carboy, so I assume maybe somewhere around half way it starts to go away. However, it is a concern knowing that each time I filter I am adding a bit of that soapy, undrinkable flavor to it. Does anyone else have that problem? I soak the pads for a couple minutes in new spring water (1 gallon) and then filter the water through for 2 minutes (using the full gallon) prior to filtering my mead. According to the directions that should take care of it, but apparently not. I suspect I could not detect this off flavor in the final product today because the trad was sweet enough, and there was enough raspberry flavor in the mel to overcome it. Im not sure if it would have in a more delicate, drier traditional though.