the other woman

i have this friend, lets call her Ana, Ana and her girlfriend (lets call her Sophia) have a really toxic relationship, they fight everytime and all of that. The thing is that, on my birthday, Ana told my she wanted to break up with Sophia, one of the reasons is that she wanted to "try new things with different people" and then she LITTERALY kiss me. During the rest of the day i saw Ana on her phone, obviously upset. That night i went to Anas house to have a sleepover and she told my she broke up with Sophia. We basically slept together. Since that day Ana has been a little closer with me. 

BUT, last weekend Ana went to a party with Sophia, and they spend the whole weekend together, and Ana added Sophia to the group chat (when they were cupple Sophia was in the group, when Ana broke up with her she took her out the group)

so honestly, i think Ana used me, like, just to not think of Sophia, i litteraly feel like im just the other woman and i dont know what to do, feel or think :(

i know im no victim and that i shouldnt have slept with her and let her kiss me, Sophia is not my friend and i thoungh "they are not together anymore and Ana said she wanted new things so is not that bad" but now i DO FEEL REALLY BAD 

any advise?