I would advise that you take a closer look at the boots you are riding in.
That is the problem an awful lot of the time, rather than the stirrups.
Many people, not saying this is case here, will try a boot or shoe on with whatever foot they happen to grab from the box, instead of making sure they are trying boot on same side as dominant hand.
If you are right handed? Measure right foot, and try right shoe or boot on. If you are left handed, left foot gets measured or the shoe or boot.
The dominant side foot will almost always be larger. Might not be more than a 1/4 of an inch, but that is more than enough to make your feet, ankles and knees ache. And in some cases, when I sold shoes years ago. There could be a whole size difference. Most generally it is 1/2 size though.
There will be a few exceptions to this rule, but when you sell "good, expensive shoes" you are taught this right at first.
And it could also be the pants you are riding in. I don't know whether you are in jeans, or breeches or jods.
But jeans are mostly made overseas now, and the whole emphasis on overseas good, is to make them as cheap as they can. If that means cutting material off of the grain, or across bias, they will do it many times.
Then when the garment is sewn? The fabric pulls your legs and bones slightly out of whack. To see what I am talking about? Take a pair of your jeans and hold them up, and see how straight, (or not) the legs hang, and if they twist one way or the other. Most will. Same with jods and breeches if made overseas.
And could be your undergarments, or the waistband constricting your blood flow on that side more too.
When you take off your boots, pants, undergarments, run your hand over the lines any of those might have left, and feel for soreness.
If your boots are genuine leather? You can wet newspaper and stuff it tightly into the boot or shoe. Or you can use plain alcohol, and take a washcloth and get it wet, and use it to dampen the inside of the leather, and then wear the boot until it dries, either of these methods will stretch the leather and may give you relief.
Or a shoe shop can do it also for a small price.
And try on boots at end of day, when feet are swollen too, as will be bigger then.
Another thing, if you are an adult and have had children, or you have gained weight, or just aged period, yet are still buying the same size shoe as you wore in HS or college? You need to have your feet measured, as all of those things will cause you to need bigger shoes. And a too small shoe can cause major problems and make you miserable.
It may be the saddle or stirrups, but I would sure take a look at the footwear you ride it too.