Help designing Butterworth filter
The first part of the response may be a result of the transient response of the Butterworth filter.  Also note that the signal is phase shifted and the basic shape is altered.  This looks like the result of phase distortion.  You may want to try again using a Bessel filter which has better transient and phase characteristics.  If you are using DSP, you can also try a true linear phase FIR approach.  This will yield the best transient and phase characteristics.