It could be a couple of things.
Have you changed shoes or type of sock recently? Or how about a change in the running terrain.
If not, your shoes may be wearing out causing more or uneven flexibility in the sole or less cushioning or a combination of these things.
The other thing it could be is a plantar neuroma. (For some explanation:
Morton's neuroma: Signs and symptoms - MayoClinic.com)
I have one of these in my right foot and it can be a b!tch. It started out of nowhere with some cramping and pain in the middle toe and progressively got worse. It's the worst when I wear shoes that put pressure on the balls of my foot or when I am bearfooted. It doesn;t take long for me to realize I am wearing the wrong shoes.
Hopefully it is something as simple a a change in shoes.
but if not, I rarely have problems with my neuroma since I stopped wearing heels and make sure the I wear shoes with a good arch support
all the time (I have inserts that I will put in whichever shoes I am wearing). But I still run and am training for a marathon, so even worst case you should be ok!