I too wore thongs from 16 weeks so can completely sympathise!
Try to stay off them -or if you are sitting down - put them up on a chair. I am a teacher and would just sit in front of my grade - feet in the air!
I also did aqua aerobics - being in cool water really helped.
Have you got a foot spa or a big washing bucket container will do- soak in peppermint and get DH to put cream on afterwards. Peppermint can be cooling and lots of foot creams have it in them so it might give you a big of relief.
Good luck!