I'm glad you two are talking so openly about this, and that you're doing what you can to be supportive! I will say the pandemic is causing a lot of anxiety, and that said anxiety can crop up in unexpected places for people. So, it could be playing a role here.
If she's working with anyone around her PTSD right now, she could also bring this up with them, as that would give her a space to process it. But in some ways, the short term solution is likely to hold off on masturbating for awhile. That might be frustrating, especially if it's something she usually enjoys. But at a certain point, trying to push through panic could accidentally reinforce the "masturbation=thing to be scared of" link in her brain.