I'm often appalled with the amount of crap people are willing to tolerate from others.
I've been going to self-defence classes for a few years. Our head instructor has a sister. She is married to an absolute bottom-of-the-barrel ar>hole. Physical and emotional domestic abuse. One time when my instructor was visiting, this guy started playing pornography on the TV while she was laying the dinner table, just to bully her. Another time he actually raised his hand at her in front of my instructor, and that time my instructor beat the crap out of him and threw him out into the street. Just one day later she lets this guy back into the house.
My instructor is well connected, he's worked with and for police, knows lawyers, and knows the law. I've called upon his help on a few occasions when in tight situations. He could have probably even killed the guy and gotten away with it. I'm not necessarily advocating for that, my only point is... he was able to offer her just about any solution to her problems that she could have asked for...
... and yet she still let this guy back in the house. Why do you think she did that?
You said that your "girlfriend" sold nudes and sexted men online within a day of her breakup, and that she told you this herself. Do you realise that she is most likely lying? She came back to you because she is emotionally dependent on you, not necessarily because she cares about you (I can't honestly tell from where I'm sat if she does or doesn't). People in toxic relationships often struggle understanding the difference between love, care, and emotional co-dependency. Often times they're so emotionally uncalibrated that they don't even understand when they're being abused or lied to (even when it is plainly obvious to anyone else).
My point - she "needs" you, which doesn't necessary mean that she "loves" or "cares" for you (she may do also, but it's not for granted). If she only "needs" you, then do you really believe that she would admit to anything more than she already has? If this risks sabotaging her chances with you?
I don't believe that sexting multiple random men online and sending them nudes, is the kind of thing that you set up in just a day. Is technically possible to do? Yes, 1. the online platforms already exist out there, and 2. a lot of men are very thirsty and it wouldn't take long at all for most women to draw that type of attention to themselves, if they choose to indulge it. And that is a choice that some women opt for (even if they don't like to admit that responsibility) and others do not. She had most likely at least been thinking about doing this for much longer than just a day. There is a good chance also that other girl friends of hers already do this sort of stuff, and slipped her the idea. There is a good chance that she already had an account set up on one of those websites, if only 'experimentally', just to test the waters and assess if it is a viable 'plan B' should her relationship fail, and there is also a good chance she was already messaging other men, even if she perhaps wasn't necessarily sending them nudes.
I've crossed paths with these sorts before (not often thank God). What they tell you is usually a tiny % fraction of what really is going on behind the scenes. I briefly dated a girl in Swindon a few years ago, before I realised she was rather 'odd'. As I slowed the pace down and our 'relationship' turned platonic, she started to volunteer some very odd information about herself. I didn't like what I heard, so I stopped seeing her entirely. A few things happened in between, but eventually it transpired she was actually a sex worker - and uh, not even a regular one. The news of it blew up in the local community and across a small number of WhatsApp and facebook chat groups. I won't elaborate further, it's not very PG.
a) If I were in your shoes, I would move on entirely and probably not date at all for 6 months just to clear my head. Spend time with friends instead. Or find new friends.
b) If I were you, personally (which is not the same), and felt compelled to tolerate this sort of relationship instead of walking away, then I would at least demand to see evidence of what it is she got up to and that she is telling the whole truth. But don't get your hopes up.