My 8 yo daughter is having nightmares.?

She's always been a bit prone to nightmares but it has been every night for 2-3 weeks. They are usually centered around zombies. No big changes going on. No scary movies, tv, video games or books. Haven't heard back from her teach yet about problems at school but she doesn't mention anything when we talk. She's waking up several times a night and even has trouble going to sleep because she's scared she'll have a nightmare. I just want to help her feel safe and get a good night's sleep!