Pain under the heel centre at the origin of the plantar fascia. This is a fan-like web of tough fibres which spreads across the underside of the foot and attaches to the origins of the toes, and its main purpose is to maintain the shape of the longitudinal arch. (This arch, along with the transverse arch which stretches across the metatarsal heads, allows proper pronation, which helps to absorb landing forces and provides some elastic recoil as your foot pushes away from the ground.) The pain is worse when running or walking, and often particularly bad first thing in the morning.
You wont experience any swelling, but press the underside of your heel and youll probably feel acute pain. A flat longitudinal arch (or a high, rigid arch both are at the limit of their elasticity) that is suddenly stretched when the whole length of the foot falls on uneven ground, can induce acute pain. New shoes or inappropriate orthotic support can also cause pain which will appear after a long run. Although x-rays may show a calcaneal spur, this is irrelevant as many that are found accidentally are not associated with plantar fascia pain, and many with pain have no spur.
What else could it be?
Other causes of the pain could be a fracture or stress fracture of the calcaneum (heel bone), bone disease and local infection, or simply bruising of the heels fat pad.
Rest aside, ice packs, good heel cushioning and a heel cup to firm up and thicken the under-heel fat pad, may relieve symptoms. Massaging the area by gently rolling the heel on a golf ball is popular in the USA. You should ensure that your shoes have good midfoot flexibility and an arch support.
Supporting a flat foot with a proper orthosis can bring relatively dramatic improvement, but you should be professionally assessed first. NSAIDs (non-steroidal anti-inflammatory drugs such as ibuprofen) can ease, but not cure, the condition. Physiotherapists can use exercises to improve the intrinsic or small muscles of the foot to ease the condition, and interferential or other electrical treatments may reduce the pain, though not affect the cause. If theres any evidence that your condition is becoming chronic, you should quickly be given an injection with hydrocortisone or a similar appropriate steroid.
Can you run through it?
If you change your shoes and use an arch support, you may find that some running is possible, but you should not run through any pain thats severe enough to cause you to limp.
One week to two years! Ignoring the condition initially is asking for long-term pain, so take steps to stop it early!