Fibula Stress Fracture Two Inches Below Knee

I am dealing with a somewhat uncommon stress fracture in my fibula about two inches from knee.  Most of the info I see if for the ankle area.  I am in a boot for 4 weeks.  I don't know why I am having this issue.  I have run consistently 160 miles a month since Sept.  I usually have 5 - 10 mile, 2 - 15 miles, 1- 20 miles a month.  There was nothing I changed.  In Oct 1st I started wearing zero drop shoes with no problems.  I have a very neutral gait with higher arches.  Dec 1st I bought a pair of Newton's and starting swapping in and out with my zero drops.  No big deal.  Calves were tired during the transition but nothing major.  I do have osteoporosis and my age is 45.  I know that does increase the risk.  Any ideas why this happened?  I have never really had an injury.  Could it have been the shoes?  Need to figure it out before I start running again if that ever happens!!!!

What mileage did you do before September ?

Probably 30 to 35 a week.  My longest was usually 13 miles on Saturday.  

So you've not ramped up things massively then ? A gradual increase ? 

The mileage went up by 5 or 10 miles a week - so not a huge increase. 

You did change the shoes too. Wht did you do that ?

Its impossible to know either way - but if somethings working - I'm loathe to change it. 

I strike on my forefoot with a toe push off, and higher arches, no pronation, very neutral gait.  October 1st gave zero drop shoes a try.  Liked them with no problems.  Balls of my feets felt much better.  Dec 1st, started transitioning into Newton Distance shoes.  Liked them. Again, felt good on my feet and keep my moving forward.  Calves were tired.  However, never any pain or soreness that I noticed any more than usual fatigue.  

you dropped to shoes without any cushioning but didn't cut back your mileage..i would guess..............only a guess that that meant the extra stress on yiour body was quite a lot from changing to the no cushioning and so your body didn't have a chance to strengthen slowly.........

maybe cut back and rebuild up slowly would have had a better effect

Thank you for the comments.  It could be.  I can't really think of anything else.  Odd that there was no warning or pain.  Frankly, this is the best I have ever run.  I felt strong and the miles were coming easy.  Just concerned without knowing the cause how am I going to avoid it happening again.