What you're describing is definitely blood pooling, it happens with a lot of conditions. I get it on occassion as someone with borderline POTS, fibro, and ME/CFS. I don't know a ton about fibro because I was just diagnosed with it recently but since all of the central sensitization conditions (like mine mentioned above) are similar I would think it's not that unusual Try elevating your legs above your heart and see if that helps, pooling happens due to poor blood volume and circulation, squeezing your leg and butt muscles can sometimes help get things moving too lol. Compression socks might be beneficial but I'd ask your doctor first since they can hurt you if unneeded or too tight