Just a remark,irrelevant with your issue..100dB for CMFB as standalone circuitry is too much,how can you realize such a large gain (which is probably larger than your amplifiers gain)?
In most cases,the biggest part of the CMFB loop-gain is located within the opamp,so the CMFB circuit as independent entity should have less gain (20-40dB let's say).
Now for your problem :
In parallel with what FvM asked you to provide can you tell us the amplitude of your input signal and also show us the output transient response for this input?