4.5 yo bedtime issues…Nightmare? Medical? Behavioral?

[das998]

3-4 days of screaming, shrieking, kicking, hitting, hyperventilating, and flailing around at bedtime. It takes 30mins-1 hour to get her to fall asleep after many times returning her to bed with her screaming at us. Sleep schedule (no nap) is bedtime anytime 6:30pm-7:30pm, and wake up is between 5:30-6:30am. This has been our routine for over a 1.5 years since she dropped her last nap. Could this be she's afraid to fall asleep? Challenging authority? Schedule needs changing? Her only consistent bodily complaint is her tummy feels funny. She's been to the doctor recently and they said she didn't appear to have anything wrong. My husband and I are extremely worried and think we may need to get more extensive medical or behavioral investigation if we can't get to the bottom of it. She's losing sleep and I'm afraid it's perpetuating the cycle of behavior, making her more miserabl