After taking my metformin for my diabetes my arms hurt and swell up a little. what could be causing that?

Could be side effect: This could be a serious side effect of the metformin, and I would call your MD as soon as possible. He/she needs to be aware of your symptoms to rule out any side effects from the metformin. Drink plenty of water. Don't ignore things like pain, because pain is our body's warning system, and pain is never normal after taking medication.