Dont want to come across as a know-it-all, or worse, an alarmist, but do your calves get really tight too? The reason I ask is that I have chronic compartment syndrome in my calves and when I exercise my feet go numb; my calves also get so tight and rock hard that I just want to grab a knife and stab myself in the legs to relieve the pressure. Back in the days after my first surgery when I'd persevere with exercise I'd get home and lie down with my feet up in the air for 30mins to drain the blood out of my muscle compartments. Long story short, this condition started out very mildly but worsened over the years and took a very long time to diagnose (lots of doctors and physios and lots of poor advice, unfortunately). Dont want to alarm you, but worth getting this looked at. Perhaps start by going to a podiatrist and see what they say; hopefully it turns out to be nothing, but would be good to nip it in the bud early. Could be you simply require a slight correction and orthotics may do the trick. Hope you get to the bottom of it quickly, I can certainly relate to how frustrating it is when it happens!!