Worried about my dog Niomi. I need advice please.

My boxer/pit Niomi is an extremely active, loving, and vibrant dog. She was fine all evening up until about two hours ago, and then all of a sudden she began acting weird. She was practically laying on top of my husband letting us to be pet her, but all of a sudden, she was extremely interested in guarding our closed bedroom door.....And now she won't leave it. I'm laying in bed right now and she is still doing it and she will not sleep. She looks a little anxious, like something is going to happen as well. She'll come over every once in a while to "check on us" but she goes right back to the door and lays down to stare through the crack of the door. She is not barking at all, nor has she. This is also the first time she has EVER done this that I recall, and it's kind of creepy because she refused to leave the door. Has this happened to anyone else before? Is there any need for concern? Thank you for your help!