Trouble tolerating sublingual methyl b12?

Hi, I'm 33 and was recently told by my doctor to take a high supplement of b12 after bloodwork showed b12 at 205 and MMA at 0.38.  This is likely due to 20 years of a vegetarian diet and lactose intolerance. Possibly related symptoms before diagnosis:  recurring canker sores, mild lethargy, lack of concentration/foggy thinking/forgetfulness, mild depression, mood swings, intermittent dizziness, eye sensitivity, sensitive stomach, etc.  I thought taking B12 was a fairly benign thing, but upon taking a sublingual dose of about 500mcg (Jarrow methylcobalamin) I developed a headache and extreme sleepiness (to the point of not being to able to do anything but lay down) within 5-10 min, lasting a few hours.  Once the sleepiness wore off, I felt like all energy had been sucked from me, and had misc. muscle or nerve pains and weakness.  My mood also went downhill and I felt like crying at the drop of a hat.  I didn't take any supplement the following day, and began to feel better in all aspects.  The next day I tried a slightly lower dose and the exact same thing happened, and the feeling of muscle weakness/twinges of pain and drained energy hasn't left me since then (I took it on Monday, today is Wednesday).  I felt far better before taking any b12.  

My tests indicate the need for b12, but I can't function when I  feel like this - and it doesn't seem like a healthy response.  Has anyone else experience this or have any suggestions as to why this might be occurring, or how to supplement in a different way?  Thank you!