Have pain under the feet while walking, what could b the reason?

Many causes. You may have plantar fasciitis, or flat feet, or a nerve problem. If this has become chronic then you need to see a podiatrist for a proper workup.
Arch problems. I am not sure of your details but if there is any weight problem the strain on the plantar tendons is great. There can be bone extending out the tendon that then gets bruised when walking. The key to the diagnosis before xray is that when you get up for the first few steps the pain is horrible but improves after 15-20 steps.
A Number Of Things. It could be any number of things, although the most common would include plantar fasciitis, improper footwear, neuroma, diabetes, arthritis, corns and calluses, and circulation issues. You need to see your physician for an evaluation.