Side effects of methylcobalamin pills

I was having lightheadedness and dizziness and shortness of breath which I thought because of my b12 deficiency but I noticed that it only happened after my pill (i tried different timings to see when symptoms onset). So I stopped eating it yesterday and there were practically no symptoms. I had been taking injections so I will probably rely on injections (once a week for 4 weeks) as my doctor prescribed and see what happens. I did check with another doctor and seeing my reports he said to have a maintenance dose of 1 shot (1mg cyanocobalamin) every 6 months and 1 D3 tablet a month lifelong.