Stress fracture/reaction - other possible diagnoses?

About three months ago I saw a doctor regarding severe pain around the second metatarsal of my foot.  When I ran cross-country in high school and college (7 and 6 years ago, respectively), I experienced 2 metatarsal stress fractures (one in each foot, subsequent years) and had low bone density (osteopenia) as a result of an eating disorder.  I had been on fosamax for awhile, take calcium, and my weight is back to normal, so supposedly my bone density is fine now.  I was also not running like I had in the past, but took regular walks and we blamed the pain on worn out tennis shoes.
Because the pain was similar and nothing showed up on the x-rays (as fractures tend not to until healing), the assumption was that it was a fracture and he gave me a walking boot.  I have had 4 rounds of x-rays over the past 3 months and still nothing has shown up, yet he doesn't want to do an MRI.  He said it is likely a "stress reaction" rather than a stress fracture which is why it's not showing up and it's taking longer to heal (i.e. less initiative because the bone isn't actually broken).  I've eased out of the boot the past week and am suddenly feeling more pain again.  My question is what else this could be or what else I could do.  He mentioned the possibility of Morton's Neuroma but said it was unlikely.  One symptom I had not reported, though, was that I often notice that the hurt foot has a more "purple" shade to it than the other foot which makes me question whether or not it could be something vascularly related.  It would make sense if there was less blood flow when wearing the boot because the weight is off it, but I've noticed it several mornings when showering (I typically swim or bike in the mornings, which the doc said is fine, but I don't kick much when I swim and this morning I swam and noticed the biggest color difference I've seen yet).  
What should I do?  Get another opinion?  Should I get my bone density retested?  Insist on an MRI?  Is there something else it could be?  Please help!