Oh my god. If you are interested in your ex, as how she is doing / if she still has a boyfriend. If you're not, don't.
Yes they might have been talking about you, and she accidentally liked an old post, or started looking as they had been talking about you, mindless bored scrolling ensued as with all of us sometimes, she saw the quote, it resonated with her, and she liked it without thinking / forgot she was on your feed.
It doesn't mean anything, it doesn't mean she fancies you or anything. If she did she wouldn't like it on purpose and show you she was deep scrolling as we all know it comes off as weird and too much, if she liked you she might like a couple new posts - but again that does not necessarily mean someone likes you at all.