Thankfully, this only happens to me when I'm pregnant.
I suggest you buy one of
these to keep under your desk and keep your feet up.
If your department chair thinks you are being high maintenance regarding furniture, and you don't want to fight that battle, just buy your own office chair. It will be well worth it.
If you have space in your office, I also recommend purchasing a very comfortable chair (perhaps a recliner) that you can relax in when you are not at your desk. (You can even buy one of
these or
these so that you can do work easily in the chair.) You can find something inexpensive if you shop around. I'm frugal, but I am wary about buying used upholstered items, but I've had great luck at "Scratch and dent" sections of furniture stores where you can just buy a brand new piece with a tear in it.