It could be, but I wouldn't worry too much at this stage.
I run a lot of trail and sometimes get the odd stabbing pain/pulling in/around my plantar fascia, particularly when running downhill.
Soreness under or around the heel could be plenty of things, it could just be your running style causing a bit of localised inflammation/bruising.
The reason plantar fascia often occurs is excessive and uncontrolled deflection of the arch. This is normally well controlled by the muscles in the lower leg/foot, but if there is any dysfunction in these muscles, even temporary, then you get a sudden overload of the soft tissue structures such as the PF.
Why this happens is multi-factorial. Sometimes its as a result of wearing orthotics and shoes with medial arch support for a long time. (whilst these can be effective in short term symptom relief, I believe an effort should be made to enhance the intrinsic support structures such as the muscles BEFORE the last resort of long term orthotics are used).
When this support is taken away you overload the PF and you know what happens. Hence why so much plantar fasciitis is caused by switching to minimalist shoes without conditioning.
What to do about it? You need to work on strengthening your feet/lower limb.
You should also see a sports doctor and have an xray if the heel pain persists as you may have calcaneal spurs.
I wouldn't ice as has been mentioned, what is that going to do?
If anything massage and heat will help deal with the inflammation in the short term, helping to improve blood flow to the area moving in good things and moving out the bad.
But this is only short term.
I wrote a short article on it here -
Just a few thoughts about PF, but to be honest it is such as individual thing I would want to see someone in person to try and get to the bottom of it.
http://www.trimechanics.co.uk/plantar-fasciitis.html
Running analysis based in Leeds UK.
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