Beta blocker stopped working after two days. Is this possible?

[whyheart]

I'm 23, female, and I suffer from anxiety. Recently I started getting PVCs and they would worry me to no end. I went to the doctor and he put me on a Holter monitor. He calls me the following night and told me that I had a 7-beat run of v-tach. I looked up v-tach and I started to get terrified thinking that I'm on death's door. I schedule and appointment with a cardiologist. I had a panic attack and another run of v-tach right before the appointment which caused me to have a high heart rate and high blood pressure during the appointment. My blood pressure is typically 110/69 with a resting heart rate of around 80-90. I had also had blood work done with all normal numbers. My total cholesterol was 149 with both HDL and LDL 69. My cardiologist told be that I was not going to die and that for some reason my high rate was high. I had an echo done right there and it came back normal A healthy heart with no heart disease. He puts me on beta blockers. Metoprolol succinate 50mg once a day. The first two days I took it, I felt great. My anxiety went down and I felt calmer. My heart rate went down to 72 and the only side effects I had were an upset stomach, restless sleep, and an occasional cough. After those two days however, the pill started to work less and less. My side effects slowly went away and my pulse started creeping up to back where it was. It's currently 84-90 as I type this. I've been taking the pill for 5 days now. Is my body for some reason fighting off the pill? Is it possible to become tolerant to a beta blocker in only a few days? I've heard of what happens if you suddenly stop taking beta blockers and I'm scared that that's going to happen to me since the pill isn't working even though I'm still taking it daily.  I don't drink alcohol and caffeine.  I don't do drugs. The only things really against me are I'm slightly over weight, sedentary, and don't drink the 8 glasses of water a day that I need.