The "weird almost tendon/bone linear line" was likely her keel.
IME, most often when they suddenly start acting lethargic, especially with females, it tends because they're having a hard time expelling an egg. Sometimes, if an egg is abnormally shaped it can apply pressure to the obturator...
Welcome to BYC. Sorry to hear Spiderman is having problems.
Could we see a few pictures of her, or better yet a video? To post a video you'll need to upload the video to a video platform like youtube or video, then copy/paste the link here.
Just a few questions I hope you can answer.
What is...