My suggestion for traveling is to be able to lie down. I found the radiation very hard all the way around
The pain can come from many different areas. I can be from the skin and surrounding tissue or it an be from the pelvic floor muscles all tensed up from trying to protect the delicate skin. I had a great difficulty sitting for any length of time after radiation. I used a "donut" and that did give me some relief. It helps, but there still is downward pressure when you are sitting and the donut can't help that part.
I was told and experienced that the radiation continues to do it's stuff for a week or two after the last treatment so things might get worse before they get better. I hope for your sake it doesn't but I suggest that you protect that very fragile skin as much as you can with aloe and anything else to keep that area (especially the gluteal fold (butt crack)) from drying out. Calmoseptine creates a barrier and heals as well. I also took a TON of baths as I found them very soothing.
Best of luck. I feel your pain. Kristi