I have a pain at the bottom of my arch in my foot. Right at the beginning of my heel. For about 3 months now. What is it?

Fasciitis. It's an inflammation of the arch -- the treatment is Ibuprofen and rest -- but you may need some foot support or local injection if that doesn't help. Please check with your podiatrist (foot specialist).
Plantar fascitis. You have excess strain on this ligament that supports your arch. You need to rest, ice, stretch and support arch to help reduce swelling and pain to this area. Do not run, you can bike and swim. You may need new better supportive shoes and orthotics. Treat it quickly or it will become chronic and more difficult to treat. Seek a podiatrist.