Which test should I ask to be done?

Hello, I have had low ferritin for as long as I can remember and had an iron infusion for the first time 6 months ago. Bloods were taken then and my b12 was at 450. A blood test taken last week showed that my b12 is now 270. As it is still within range, my PCP won’t mention it so I need to bring it up and want to make sure I ask for the right tests to be done. I have had gastrointestinal issues lately which I don’t know whether they could be a cause or symptom. It’s all new to me and I don’t want to miss something important. Thank you