Hi.
In case of repetitive strain you need to give rest to your muscle for sometime. You can carry on with your routine work but avoid heavy workout or sports.
Your muscle in this case is already tired and working more on that will worsen the condition.
Next Steps
Rest
Iceing
Isometric exercises for hip flexors. (search on net)
strengthening exercises for flexors of hip and gluteus (hip) muscles will help.
Health Tips
Avoid sitting on floor
Always sit with leg supported on floor or table.