Konzy.... if you react to the COMPLEX ... that is pretty common.
It would be based on the dosage you are using.

Some Complexes are moderately low dose... like Jarrow's B-Right.
Others are high like B-100's.

I would get the test results you had previously from your doctor, so that it would be more accurate to advise you based on the numbers. (tests from other countries often have different reporting results ..in US it is pg/ml... so get your numbers and the letters when you ask for your test. There is a conversion table to convert other concentrations to pg/ml.)

Modifying your dosing...to lower milligrams, or taking every other day...may help with your symptoms.

But really you need testing... and if you do get more blood work, also get the B6 measured. It is usually that portion of the Complex that bothers people.

If you are very low and start just B12... you may feel energized at first. That would be your body trying to heal itself. Taking it every other day may help. Is the product you are taking cyano or methyl form? Some people cannot handle the synthetic cyanocobalamin as it contains cyanide. The other forms often work better.

Also people with heavy metal poisoning (mostly mercury) may have symptoms/side effects from methylcobalamin. You would need to check that with a heavy metals blood screen. Heavy metal poisoning (from the environment) can be treated with chelation to remove them.