if you get worse taking D3 then i'm even more suspicious that your magnesium is down.
D3 will use up some of whatever dietary magnesium you get, in order to absorb properly. if your dietary intake of mag is low to start, and you supplement D3 without a balanced mineral supplement, then you can throw things out of whack.
as for neurontin it's a bit of a nutrient drain:
nutrients depleted by gabapentin
if it's used to relax muscles, there are two major constituents of many antacids which also affect muscle contraction and relaxation. calcium helps muscles contract, while gabapentin depletes calcium, so if you mixed those two you'd be canceling out the calcium reduction. magnesium is another ingredient in antacids and this would help muscles relax, so maybe if you took magnesium with gabapentin it would work too well. not just because magnesium helps relaxes muscles but also has links to GABA release. your body is supposed to release GABA on its own and there are links to ATP.. specifically magnesium-dependent Mg-ATP.
wikipedia: "Gabapentin (brand name Neurontin) is a pharmaceutical drug, specifically a GABA analogue."
this next bit is not properly referenced but it's worth a read. i took a couple stabs at locating some abstract links, to back these statements up, but i don't have all night so this is it for now
"Magnesium is a critical element in 325+ biochemical reactions in the human body.
Recent research, in France and several other European countries, gives a clue concerning the role of magnesium plays in the transmission of hormones (such as insulin, thyroid, estrogen, testosterone, DHEA, etc.), neurotransmitters (such as dopamine, catecholamines, serotonin, GABA, etc.), and minerals and mineral electrolytes.
This research concludes that it is magnesium status that controls cell membrane potential and through this means controls uptake and release of many hormones, nutrients and neurotransmitters. It is magnesium that controls the fate of potassium and calcium in the body. If magnesium is insufficient potassium and calcium will be lost in the urine and calcium will be deposited in the soft tissues (kidneys, arteries, joints, brain, etc.).
Magnesium protects the cell from aluminum, mercury, lead, cadmium, beryllium and nickel. Evidence is mounting that low levels of magnesium contribute to the heavy metal deposition in the brain that precedes Parkinson's, multiple sclerosis and Alzheimer's. It is probable that low total body magnesium contributes to heavy metal toxicity in children and is a participant in the etiology of learning disorders."
i'm a greens lover too! but i still have to supplement magnesium and a few other things, in large part due to the d3 supplement.
hope that helps!