Difficulties at work

I work 8 hours a day, on my feet all day, and I've started to get a bad back and feeling dizzy occasionally. I've been nipping off to sit down every few hours (with permission) and some of the people I work with have started making snide remarks about how I never do any work, don't deserve to be paid and "don't need to sit down just choose to". It's really getting me down! Feel like such a wimp but just want to do what's best for me and baby! Anyone else having similar problems?