I comme to that conclusion because it only happens when her cum is in contact with an other girl's vulva, vagina or uterus. (It is unclear at this point what organ need to be reached but certainly not the mouth)
Is that why paige said her mouth feels "tingly" after jenny came in it in the dressing room?
She tested that by drinking the slushie jenny came in and have jenny drink it as well.
She felt it, jenny did not.
Meaning jenny is immune to the effects, since that usually is associated with poison I named it that.
Ashes test revealed nothing because some compounds dissolve fast in the human body.
Remember, she tested the cum from her vagina and not "at the source" either.
For instance that is why common salt would be an effective poison were it not so salty. It dissolves in a matter of hours in the victims body. Similar with nicotine ((smokers please raise your hand now!) but the lethal dose is waaaaay more than in a packet of cigarettes).
To be clear, paige seemed to have the effect the girls jenny came in vaginally as well, just not as "bad".
My guess would be that the part of the body need to be reached is the blood stream.
The vaginal walls are rich in membranes directly linked to arteries.
Way more than there are in the mouth.
Meaning the effect is faster and delivers a shock so to speak while it is slower orally so the receivers body can adapt.