Question About Working While Pregnant

So, this is baby #4 for me, but I was a SAHM with 2 of my others and had a desk job with the last one, so was never on my feet. I've been at my current job for about 2.5 years. I'm used to spending 8-10 hours on my feet, no problem. But since I've been pregnant, I barely make it to lunch before I'm ready to collapse from the pain in my feet. I'm 20+3, and have only gained 9lbs, so I don't think it's the weight (plus I lost 30 pounds in the last year and a half before getting pregnant and was at my ideal weight prior to conceiving). I plan on getting some shoe inserts today, and my fiancé rubs my feet nightly after work and it helps, but then I go to work and the pain is right back. Is there anything else that has worked for you? I was thinking about getting a foot spa and some epsom salt, but am reluctant to spend the money on a good one if it's not going to help.