Lanacaine is an effective 'anti-itch' cream, containing a local anaesthetic. You probably have some sort of photosynthetic reaction (ie reaction to certain types of light - UVA/B), and you might need to consider using some type of barrier cream in the form of a total sunblock, or even consider seeing a doc for a steroid cream for short term use. Note that the steroids on sale over the counter in the UK eg Hydrocortisone 1% may not be strong enough for your needs.
If you are holidaying in an area where anti-malarials need to be taken, you may have a reaction if you are taking DOXYCYCLINE.
I'm not a medically qualified person, but suffer from the same rash in the same place, and also have eczema and psoriasis (lovely cocktail that!!). Do please see your doc!
As for the chaffing thighs, there's not probably a solution, because we all carry our fat differently!! When you change out of your costume, might you consider wearing cut-off tights (waist to knee) to separate the two rubbing surfaces, and you could put talc on when the thighs are DRY to increase friction between the two surfaces.
You have my sympathy because it's not fun! Moisturise this area well at night anyway and apply a calming lotion if you do have sore bits there.