If you recall, it was mentioned on page 008 that Selina was created from a drop of Vellian's blood and clay from the cavern. On the next page 009, I said in the author's comments that the Clawfoot took on the traits of the dragons that used to live in the caverns, because trace amounts of the dragons' blood soaked into the ground and their food, shaping the Clawfoot over time.

The clay that was used to create Selina had traces of dragon blood, too. So when Vellian attempted to kill Selina by reverting her back to her clay form on page 042, it allowed Selina to get in touch with that dragon blood that had stayed dormant inside of her.

The actual science behind the magic (not a phrase I ever thought I'd get to use!) of Selina's healing factor is that every time she shape-shifts, to human or dragon-form, her molecules reset. She will still age normally, but any minor (or major) injury, sickness, or blemish, can be erased as long as she can transform!

Woo, sorry for the wall-of-text! But I want to give a shoutout to Crystal-smith who put all the pieces together on Deviantart a month ago and nailed exactly why Selina can turn into a dragon. Bravo! Stellar deduction skills, Crystal! :D