Great picture! That definitely helps to localize your area of pain. It could be a nerve impingement, but they typically occur more on the top of the foot. I would expect you to have some discomfort (usually in the form of burning, sharp or electrical pain) while training for the triathlons - either running or biking.
nerve-impingement-area.jpg
Considering you've been training and competing in triathlons the last 2 months (and playing footy the last 15 years), it is more likely a result of the triathlon training, and you are experiencing the pain more with kicking.
For nerve impingements, place
padding around (not on) the area of pain and lace your shoes differently.
Tips for lacing shoes.
But, more importantly is that you are running in the right pair of shoes. Shoes which are too flexible will cause foot problems. More on
choosing shoes.
The area of your pain is at an area where the foot transitions from the rearfoot to the midfoot. Shoes that are not supportive will place excess stress in this area. The anterior tibial tendon travels where your pain is occuring and attaches on the top of the midfoot area. This tendon lifts the foot up (
dorsiflexes the foot), and could be overworked if you are hill running. This tendon would also be one of the main stabilizing forces while you are kicking the ball.
Pain from the tendon could give you the initial sharp pain when you are kicking the ball. The pain would most likely be from the ball forcing the forefoot down and the tendon unable to stabilize the forefoot on the rearfoot. Stiffness and dull pain would follow, and then it would work itself out. Treatment for this would involve backing off on any hill training or running on uneven surfaces and a making sure you are in a stable pair of shoes.
It's difficult to know exactly what the cause is, without an examination. If the pain continues, you should visit your doctor. Hopefully this helps.