I'm afraid she's jolting herself awake as she's going into the deeper sleep cycle hun. She won't wake up whilst your holding her as you're essentially holding her through the jolts (HTTJ).
One things to try is applying light pressure on her arms and legs (tricky to do with only two arms) around the 15mins mark, wait till you feel the jolt and ssh softly until you feel her relax again. Remember 3weeks is so very little, it will get better hun. You're doing so well already actually getting her to nap on her own, amazing in fact xx