Is heart reaction flirting?

The reaction on its own tells you very little, so start by looking at the message it landed on. If it's a joke, a photo, or a comment about something you both did, a heart is a warm and easy response that costs her nothing. If it's a question you asked her and she hearted it instead of answering, that's a different story and usually means she either didn't want to write back or was distracted. Check how she normally reacts. Some people heart every message from everyone. Others almost never use reactions at all, and for them a heart is a bigger deal. Also look at whether she initiates. A woman who has never started a conversation but hearts your stuff is showing interest through low-effort signals, which is fine, but it means you should watch for whether she follows through with real messages.

You're trying to figure out the level of effort she's willing to spend. A heart is the lowest level on that scale. The question is whether she moves up from there when you give her something to respond to.

Your reply should ask her something small and specific, or add a detail that makes her want to add one of her own. Short replies that just acknowledge the heart tend to end the thread. A reply that picks up the same thread and adds a new angle keeps it going. Think about a message that says something about your own day or your reaction to the thing she liked, with a question tucked at the end that is easy to answer in one line. The job is to make the next message feel natural for her to send. Avoid anything that asks her to explain the heart, rate her interest, or commit to a plan. Keep the energy light enough that answering costs her almost nothing, and let her decide how much to put in.

Wait a reasonable amount of time before you follow up, usually several hours at minimum, and ideally until the next part of the day when you'd naturally pick up the phone. Replying within a few minutes can look like you were waiting by the screen, and that lowers the perceived value of the conversation. If she answers, match her pace roughly and let the thread breathe. If she doesn't reply to your message at all, that's a clear answer that the heart was the whole conversation for her, so don't send a second follow-up asking why. One gentle check-in a day or two later is fine if the first message was warm and interesting. After that, let it go and move on to someone who engages back.