This is something that derailed my 2017 base build-up. It all started in July. I started feeling more tired than usual. It was nothing new for I was running 60 to 65mpw with 3 to 4 days of doubles. I was building up base mileage but only taking a day off when I felt like I needed one. Come late July I started to get some residual soreness along my fibula not really giving it much thought I figured It'd go away just like all the other aches and pains that come with running.
I started up phase 2 of Daniels which is short speed on the track lots of 200s and a few 400s. My mistake was not taking a week off OR reducing mileage prior before transitioning from base miles to phase 2 in Daniel's. By the second week of Phase 2 that residual soreness amplified along my fibula. I soon realized it was no minor ache any more. I took off a day being super optimistic it was just a tendon problem.
After taking off I day, I went out for a run. Something didn't quite feel right in my leg. It wasn't painful but something wasn't right. I continued to run that day. Put in 14 miles and went home. My leg still wasn't exactly hurting on a pain scale but more like something tingly about it. The next morning I could barely walk.
I ended up having to take 9 weeks off from running and I lost a ton of fitness. I returned to running the first of October and eventually got my miles back up to about 50mpw in December.
I do not know what exactly happened to my leg. I failed the hop test. My leg ached but DID NOT feel like a sharp pain so it might have been a stress reaction. During my recovery I iced, rolled with the stick, stretched, and strengthened my peroneal tendons in hopes that would help. I ended up ordering a pair of super marshmellow shoes aka Hokas and started running again in those, which helped a ton in my return. I used to train exclusively in Kinvaras for everything.
I don't know if any of this will help you. I never did see a medical professional due to the obvious idea that they would just tell me to rest until I felt better. I knew if I did have a stress fracture it wasn't going to show up on a basic xray anyways, and I was not to pay $$$$ money for an MRI just to confirm my suspicions about a borderline sfx I believed I had.
Takeaway: listen to your body. Any little ache or pain needs to be monitored before it gets out of control. Take days off if an minor ache os starting to get a little worse. Don't transition training without adequately thinking about it. Consistently work on muscle building in the kinetic chain. Monitor the wear and tear on shoes. If you're rough on shoes (I am) then replace before they breakdown too much. Like I said, I don't know if this helps but this was my unravelement with this type of injury. Could be something super minor but quickly become major if you're not cautious.
One stretch that helped me was to first put on shoes. Next, stand up. Now roll your ankle on the affected leg to apply pressure to the fibula. Now bend forward and try to grab your ankle on the affected leg while twisting your body ever so slightly. You should be feeling a major pull or tension in the muscle of the rolled ankle leg. Be careful not to over do it.