This link describes various imbalances in some metabolic systems and how they make people feel.
http://www.alternativementalhealth.c...s/pfeiffer.htm

When people get very constipated as you describe, that suggests low serotonin. Taking B6 may help you along with zinc.
Pyroluria is what causes very low B6 and zinc in the body.

Try P5P for your B6 if you can find it. One a day
http://www.amazon.com/Vegetarian-P-5.../dp/B0009P3DT6

You could also go to a naturopath or integrative doctor and have some testing, for a precise answer. But when the neurotransmitter synthesis gets imbalanced, people report all
sorts of uncomfortable type symptoms. Serotonin is the main neurotransmitter in the GI tract. Too much causes fast transit and diarrhea (often a side effect of SSRI antidepressants) and too little causes slow movement and chronic constipation.

A good zinc supplement is OptiZinc. Not expensive...and do
one a day for 2 weeks. If these moderate doses help you, then you need to find a doctor who understands pyroluria and can dose you appropriately --usually higher than what I suggested.

Do you know what your B12 test was before you started B12?

Some B12 will mobilize heavy metals, from storage in the body.
If you have a mercury load, then taking methylcobalamin may cause side effects as the mercury is excreted. This is really not
common, but you can get tested for heavy metal toxicity.

Soaking in an epsom salt bath will also be calming if you are low in magnesium. Magnesium and B6 are the cofactors your body uses to make serotonin. So if you find epsom salts baths
helpful, then taking a good magnesium supplement like SlowMag will help provide those cofactors so you can improve your serotonin status.

What is happening to you is complex, and perhaps seeing a doctor who understands these types of metabolic systems, will be of help for you.