Why does caffeine give you the jitters?

I am on Metoprolol twice 25mg a day for a First time A FIB episode Oct 29th. never felt a shakiness inside before. It's like a jittery feeling inside my chest. my heart rate however will show 62. Can it be a vitamin deficiency? Can it be the med itself? I dont evenhave to drink coffee to feel this way. This medication is for High blood pressure and angina. I didnt have either of this before. Now i am experiencing a dull ache on my left side off and on and an increase in my blood pressure to around 130 to 140 / 86-105. My doc told me to increase my medication to double 50mg twice a day. I have done this for 2 days with no difference. Just curious if anyone has ever asked this question