She may not be "lying" at all. She may think she really did stay in her bed all night -- after all, that's where she woke up. She is eight, not eighteen.
This is likely a phase; you say it's been going on for about three weeks consistently; that is really nothing overall in a kid's years of development.
Just because she cannot articulate whatever fears or anxieties or just plain need for closeness is sending her into your bed -- she still may be feeling those things, even if she can't articulate them to you as you'd like. You say you've tried asking her about bad dreams, etc., but do you ask in ways that make her realize you are probing so you can "solve" things and get the desired result: Evicting her from the bed? I bet she is aware, on some level, that your attempts to talk are attempts to get her into her own bed in the end. So she doesn't respond as you would like, with easy-to-solve problems you can identify and eliminate, and you get more frustrated with her. It's understandable, especially if you are losing sleep, but you might be expecting more of her than she is able to do at her age.
Her brain is still developing so fast. You have no way to know what's going on inside her head or with her emotions when she wakes up in the night and needs you. Sure, it may be as simple as she's afraid of the dark -- and she's afraid to tell you so because it will make her seem babyish to you. Or it may be as complex as a new fear of being alone, or of waking up to find you're gone forever. Nonsense? You know that, and if you asked her, she would say, "Oh, of course I know you'll be there." But what her logical mind might "know" and what she might feel at 1 a.m. are two very different things at this age.
Please don't assume that she is intentionally lying to you and intentionally manipulating you to stay in your bed. She needs you. The reasons why aren't clear to you and certainly may not be clear to her either. But why does it bother you so very much when it's gone on for this short time only? Maybe take a look at whether your expectations of her, at age eight, are expectations you'd have of a somewhat older child. I'm not saying baby her; but do cut her some slack if you can.
If you just can't live with her sleeping in your bed right now, then do what you'd do with a wandering toddler: With minimal talk but gently and kindly, walk her back to her bed if she comes into your room (you'll lose sleep getting out of bed to do it), with lots of reassurances that you're around. You might even have to stay in her room a few times until she falls asleep, if she comes to you in the night and you have to walk her back to her room. But locking the door on her every night as someone iin a post suggested? To me that sends the message to a child, "You're on your own, in the dark, all night; don't depend on me to be available." Not a message that builds a child's confidence in the parent.