Another thing effecting training :@

[AdamGB]

Could be caused by tight fascia, try a foot roll (you can use dumbells) on the arch to release it, if it's tender then that's a sign that it's either knotted, inflamed or both - either way rolling and icing will probably help as would trying to relieve any knots on your posterior lower limb muscles.

Try foam rolling.

Have you had somebody assess your gate? you could be over or under pronating or something.

This may sound slightly stupid - I had a personal training client, he was an older guy and was trying to get his fitness back (he was a football referee, full pitch 90 minutes). He was complaining to me of tenderness in the arch of his feet and instability in his ankle. He worked as a security guard to he spent a lot of his day on his feet obviously during football he had appropriate footwear but I learned that he had started wearing new work shoes - they were a really loose fit on him.

Basically from what I guessed the stabalising muscles around the ankle joint (that insert into the fascia under his foot) were getting overworked because of the instability loose shoes were causing (especially seeing as he was walking up stairs a lot at work - plantar flexion from the soleuos on an unstable surface) I gave him some exercises to strengthen the joint but what really sorted it (once I'd cottoned on he'd changed his work shoes) was getting smaller shoes and ensure that they were laced up properly...

It's unlikely, because you're younger but try making sure that your training and everyday foot wear is tight enough.