If I'm reading this correctly, this itchiness only happens in ONE particular area...........that being where the panty line hits the hip area. Since you've apparently ruled out the type of panties, the material, the detergent, the softner.........saying it's the same for ALL your panties, there's only one thing I can think of. Knickers are sewn together with elastic thread. Elastic thread is often made out of latex. I'm wondering if you have an allergy to latex?
You may or may not find this experiment gross or disgusting, but personally I'd try going cammando for a few days and see if the problem disappears. No panyhose either.
Why so adamant about no dermatologists? There ARE women dermos now, if that is your concern.
Best of luck!
Peace
Greenlydia
Might be dry skin probs. Sometimes the combinations of the friction, material, dyes, laundry stuff, & dry skin can cause the itchies. I would use a fem wipe then add some good lotion (the 1s w/out perfumes, like Eucerin cream) & see what happens. Good luck.