As Mrs. D pointed out, that is not at all common behavior for plantars, you seriously need to get to a doctor to find out what exactly it is. I have tons of joint problems from my spasms, it's caused a multitude of tears of tendons and ligaments as well as bone spurs and minor fractures (stress fractures) to various bones. Its highly painful, causes a lot of swelling, and happens fairly regularly. But even if I'm pretty sure it's just yet another torn ligament or stress fracture, I head off to the doctor to get an MRI of the area to find out "what now...." It can be pricey depending on insurance, but it's always better to know what's going on and to hopefully be able to correct it.