Feet are killing me!!

Okay, so, last Thursday I had started a job that is a 12 hour shift, I had never worked that long before, especially standing that long.. Well, I know my body has to get use to it but the heels of my feet are killing me, sometimes they feel as if they're on fire or have a Charlie horse (foot cramp in my heel) I just bought a new pair of shoes (sketchers cooled memory foam, light weight, fit perfect, comfortable), but my feet still hurt, so I bought insole things for just the heels of my feet, used those today. any pointers/tips to help improve? I've worked last Thursday through Sunday, off for 3 days now I'm working 7 days in a row (tomorrow will be day 5 out of 7). Oh, I also bought a bubble foot spa and user that with Epsom salt tonight.