Since it is more in the heel and you do have flat feet I would say more likely to be a bone spur rather than gout or peripheral neuropathy.

Bone is a piezoelectric material.  Therefore when it is exposed to stresses the bone will generate electrical currents that electrodeposit the minerals in to the collagen matrix of the bone.  When one are is constantly stressed the bone growth is stimulated at that point.  So how does this apply? When your foot arch goes flat it pulls the plantar fascia tight.  Since this is connected to the back of the heel this causes a tugging on the back of the heel when you walk.  As it tugs the piezoelectric effects starts to produce excess bone at that connection point.  This is the spur.  And this spur can cause a lot of pain when you walk.

I don't know much about the master cleanse, but from what I recall it uses cayenne pepper.  Cayenne is a pain reliever due to both its stimulation of substance P, and because it is nature's highest level of natural aspirin.  So I think the cayenne has more to do with it than the lemons.

I doubt if colon problems are contributing.  Were you more active when in Mexico, maybe walking a lot more down there?

Caffeine can definitely lead to increased inflammation.

My recommendations to try are to get orthotics for the arch support, and use the silica and vitamin C to strengthen the plantar fascia and to reduce inflammation.  If it is a spur, and you take the pressure off the bone the body will eventually reabsorb the minerals from the spur.