Nothing as major as that. Last week Sassy and Cookie were playing and Sassy went for Cookie’s tail. Cookie, being in season, was a little sensitive about anyone going for her tail and read Sass the riot act. Fortunately, Sass just froze submissively like “I don’t know what I did but I’m sorry”. I had to tell Cookie “enough” and then they separated themselves for a couple hours but were fine after that.
I know you said Molly’s been having some pain - perhaps the other dog accidentally did something to hurt her and she snarked? Keep a close eye on them - you don’t want it to escalate. For now at least, I would keep them separated when you can’t keep an eye on them.