I had a very similiar problem to yours. I finally tracked it down to 3 things. The first was the type of soap that I used. Dove, Caress, Tone and many of the so-called mild soaps irritated the crap out of me so I had to go thru a grueling trial-n-error period exploring almost every soap on the market including Cetaphil. Cetaphil helped the first week but the itchy irritation came back. I finally discovered that Safeguard was the ONLY soap that didn't cause problems.
The 2nd source of my problem was the type of pads and pantyliners that I chose. Again, spending lots of money trying every brand on the market until I came across a brand from a popular online drugstore which was supposed to be "all natural". Well they worked for a couple months but the irritation came back. I finally came across Stayfree pads which has worked wonders.
The 3rd source of my problem was the elastic on certain underwear. I had to throw out several pairs until I found some that didn't irritate.
Finally, after seeing a dermatologist, she diagnosed it as a form of eczema or contact dermatitis. She prescribed a steroidal ointment then promptly gave me a few samples of Dove and Cetaphil soap. (I cringed!!) The ointment helped but the eczema returns periodically just as she warned. The flare ups aren't as bad these days since I've changed the products that I use.
Like you, I read everything I could find. What I found was that some of the least suspecting items can cause problems such as pads, condoms, lube, sex toys, elastic on underwear, soap and even toilet paper! I'd suggest trying different products to see what works for you.
I hope this helps. Take care