What Causes Stomach Cramping And Burping After Taking Metformin?

I have been trying to get my diabetes under control and have been using metformin. Several years ago I tried to use it but had severe cramping, diahrea, and nasty burps so i went off the drug. Now I am trying it again. At first 50 1x day with no side effects, then 50 2x day with no side effects and then 100 2x day. When I starting taking the last dose I started getting the side effects again. I stayed off it for several days and then went back to 50 2x day. I still have some side effects although not as bad with 100 2x day. Is my body just rejecting now? Do you think the side effects will go away or should I just try something else? The stomach cramping and the disgusting burps are the worst! Also, I am using metformin from several years ago as I had just gotten 3 months supply when I went off. Do you think it being so old could give me these symptoms worse than if I had gotten new?