Are you 100% sure it's a foot? If it's always on your right side, it might be your gallbladder.
When I was pregnant with DD I thought her foot was constantly up under my right rib because I had so much pain & pressure there for my entire 3rd trimester. But with this pregnancy, I had the exact same pain, only it started earlier, and I knew it couldn't be a foot yet, not up so high under my ribs.
I got it checked out, and apparently it was an inflamed gallbladder -- fairly common in pregnancy (which I didn't know until my MW told me) and quite painful. I modified my diet (cut out almost all fat, except a little bit of nuts, avocado and olive oil), and the pain has gone away almost completely. If I slip and eat something fried, or have some cheese or pizza or something, then the right-rib pain comes right back, but the modified diet has helped a LOT.
Just wanted to be sure you considered this possibility; I'm sure it was the same thing with my first pregnancy and I never realized it. If you're having really bad heartburn and gas/bloating in addition to the rib pain, I would definitely look into gallbladder as a culprit. I can't believe how much better the heartburn is since I went on the gallbladder diet. I can even drink coffee this time around; I couldn't even look at the stuff during my last pregnancy without having severe heartburn.
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