I just did 25 days of the Arise & Shine cleanse, 14 days of mildest, the rest gentle. At times it was uncomfortable, although I haven't had your particular difficulty. I would ask you this:
Are you staying strictly alkaline with your diet?
Are you drinking enough water?
Are you able to juice leafy green vegetables? This will help if you can.
Can you take enemas? If you haven't considered it, I would consider it. Dr. Anderson recommends it, and it helps the toxins leave your body faster. I was new to cleansing and enemas, and I found the enemas to be relaxing (I do them in a warm bath).
It does sound like a healing crisis to me. How much external stress do you have going on right now? If it seems to be going up, there might be too much stress for you to cleanse (my health care provider, from whom I purchased the kit, advised me to wait until a low-stress time in my life, said it was really important).
After all the days I was on the cleanse, I passed a lot of garbage, but no mucoid plaque. I guess I'll have to wait 'til next cleanse. Dr. Anderson DOES say that not everyone passes plaque the first time, that it might take a whole cleanse just to prepare it to be expelled.
I would try all of the above, and make sure you're getting extra rest. Also, gentle exercise is important. If you don't already have a yoga practice going, I'd take a couple of walks every day, breathing deeply as you do.
Let me know how it goes. I'm rooting for you!