Thanks for posting your question. If its only sitting on hard objects that is causing pain and no pain otherwise, it is quite likely that the pain could be because of bursa (soft-tissue) inflammation on the outside of your hip, most commonly Ischial Bursitis. However, a complete hip examination may give more information regarding the inside of your joint.
Next Steps
It is best to try and avoid to sit on hard surfaces as pain and inflammation can exacerbate. If there is too much inflammation, ice application may be useful. If severe, you may require anti-inflammatory medications. You can use soft pillows at home and portable inflatable cushions outside. It is best to avoid prolonged sitting in one place for a long time until pain subsides.