Hmm... its probably from the flat feet, but if you get a chance to ask the doctor, go ahead. Maybe look into getting some good inserts for your shoes. I like to use sof sole inserts, but not the orthopedic ones. In your case, gel inserts might even be better, or an insert with good arch supports. I got mine from Amazon, except my gel inserts I just got from Walmart. Hope this helps!

We cannot tell you what it is without doing some kind of exam.

Go to a doctor.

You do NOT have plantar fasciaitis

Probably not. The tell tale sign of plantar fasciitis is pain on the bottom of foot when stepping down on it first thing in the morning. Sounds like maybe a nerve is pinched.