Hip-flexor hurts when doing cartwheel. What does this indicate?

Many possibilites. At 31, it commonly is a strain, if it just happened. If it's been there for a while or has been happening over and over, then it could be several things.. A hip flexor tear (rectus or iliopsoas), capsule tear, hip labral tear, arthritis, cam lesion, early avascular necrosis, or stress fracture. Unless it just happened today and you're walking without pain, i would see an orthopedic surgeon.