Pain in shins and ankles...what could it be?*endurance riders!! i need your help!!*

Ok guys, now, I am an endurance rider. I went training yesterday afternoon, and after a night out, I went to bed. At about 3:15 in the morning, I woke up with a terrible pain in my shins (i believe you call them 'shin splints' for thouse of you who do track and field. ) and ankles. I was lying half awake in bed, moving around constantly to try to ease the pain.

I finally got up and plugged in the heating pad. The problem was, keeping still made the pain alot worse. I went downstairs and took 2 tylonol, and after about 20 minutes, I felt much better. I finally was able to sleep again at around 4:15AM.

Anyway, when I go training, I don't keep my stirrups very short. I prefer them to be +1 hole when I ride my horse as opposed to others (mainly because he's so narrow in the front and so wide in his hind end). Either way, they are at the same angle all the time.

Now with the stirrups. I can't tell you the exact brand, but they are metal endurance stirrups with thick padding. I replaced the pads recently, and they are not the E-Z ride brand (the thick, spongy material) they are very solid, but still shock absorbing.

I have a feeling it is these endurance stirrups. I've ridden in irons my whole riding life before I came to this barn. I don't remember getting these shin splints before then.

Im worried about using irons long distance. A girl who used to race for us did, but unfortunatly I never get to see her because she quit.

Does anyone ride in irons? Are there any endurance stirrups for this problem?

Could this be the problem? Or should I teach my self to ride longer?

Please help! My legs are killing me. :[

Thank you guys so much!