I believe I had plantar fascitis (sp?) before I started my weight loss journey. I am 99% sure it came about because of my weight gain (gained 60 lbs in 1 yr)!
I started running at about 200 lbs...and I'm bottom heavy (meaning I carry most my weight in my thighs, hips, butt and legs), so running is even more of a challenge to my joints and feet. At first, every morning that I woke up, I would limp around for about 10 mins and after each run, which also be limping around. Eventually, I went to a running store and got fit for a new pair of $100 running sneaks (saucony progrid guide 3)...foot pain quickly but gradually subsided.
Not sure if it was the sneakers alone, or the fact that I lost alot of bodyfat...but I no longer have that problem anymore.
Every case is unique, so as everyone else has said, the best advise is to see your doctor. But if your like me...and won't go to see a doctor (stubborn, hardheaded, broke, etc.) looking into a new pair of sneaks may be a cheaper alternative.