I came across an image on Deviantart that has Japanese text written on it, so I am trying to translate it. You can see it here: https://www.deviantart.com/ryonapage/art/Dominated-710481249 (Warning: Not Safe For Work)
The part that I'm stuck on is the onomatopoeia ピクンッ. I found out from the website (http://thejadednetwork.com/sfx/search/?keyword=%E3%83%94%E3%82%AF&submitSearch=Search+SFX&x=) that ピクッ means twitch. But it doesn't have ピクンッ listed. Is it some sort of variant?
If you know the answer, please tell me. Thanks in advance.