It is difficult to say. The B12 does energize some people.
If you have an underlying cardiac condition, it may be too much.

Stay off the B12 for a week, and see if you get the palps any time during that period. Take one tablet (swallow) and see what happens. If this is the 5mg, try cutting it in half?

I do see my cat Oreo getting pretty lively on her new B12 regimen too. She was tentatively diagnosed with lymphosarcoma yesterday at a tertiary vet clinic. Samples of cells were aspirated from the tumor and sent off. Her prognosis is grim now.
But the vet there said to continue her B12 as I am doing. 5mg methyl orally twice a week. She weighs 8 lbs. The vet had heard a murmur before the B12, but now the murmur in the heart seems silent. She seems more involved in life since I started her. (she had a parasite treatment, and they think her intestinal absorption of B12 was impaired by that).

The heart will race when your blood pressure falls. That can happen with some blood pressure drugs, esp the ones that dilate blood vessels. This is a compensation for the falling blood pressure.

But it is hard to say what is going on with you. You may only need B12 2 or 3 times a week, or not at all. You may not need 5mg either. So go a week without and see if you have the palps.
If not if you choose to restart start at 1/2 of what you were using before, and see what happens.