Of course, if you ride a recumbent, you have superbly developed hip flexors as they are used to lift the near horizontal, extended leg in a way not really comparable with riding an upright.
It's the main reason why fit upright riders jump on a recumbent and ride hard for about 30 minutes before the pain get to them
If you have flexible hamstrings, Kirstie, you'll need to be especially careful keeping the hip flexors loose too otherwise (as you probably have found out) they can cause lower back discomfort. My physio says it's a common for women with flexible hamstrings getting sacro-illiac problems (rather than lumbar spine disc problems) due to tight hip flexors pulling the pelvis out of alignment.