Indeed, your probe's impedance and reactance is so similar to the impedance of the signal load you are measuring on (15pF cap) that it is significantly affecting the waveform.
With these sorts of measurements, you may have to resort to an active, FET input probe for a more accurate image. The lead length and grounding tips mentioned earlier are good to follow and may help you to get closer to not affecting your measurement with your measuring device, but these sorts of sensitive measurements are often done with the active FET probe to minimize measurement error