Clenching to hold it in makes me feel terrrrrribbbleeee! Why?

Yesterday I was driving home and got the urge. I was literally 2 minutes from my house, with no bathrooms on the way. I clenched as hard as I could, didn't make it, and you know how that story ends. Immediately afterwords it was like my body had shut down: I got terrible pain all over my body all the way down to my bones, shivers, feeling like I had a temperature. After a long hot shower I lay in bed under a blanket with my heating pad until I finally broke down and took a Tylenol. That was the only way I was able to function and feed my family dinner.

Today the same thing: had to clench on the way home from a long car drive. Two minutes away from home accident again, with the same result.

I hate taking Tylenol as I just want to give my colon a break from anything harsh.

Any idea why my body does that? Any body else go through this? Could it be the strain I put on myself trying to clench (I'm very skinny but terribly out of shape)? Shouldn't it just get over it? Could this be mental?

I'm definitely flaring, been in a bad flare since April 2010 (I don't know why sometimes my siggy comes out and other's it doesn't). I'm on 1 mg pentasa and starting a new homeopathic regimen, after trying 4 mg pentasa, mesalamine suppositories and enemas, and going up to 40 mg steroids.

I don't feel like I've got anything else going on.....