流量是plan，想不到吧O(∩_∩)O~

“流量” in English is either "flow rate" or "flow, amount of flow, flow quantity", depending on the context. With reference only to mobile data, "流量” is either "套餐流量” == usage allowance, plan data limit, or “已使用流量” == actual data usage.

To be really correct, you need to include the word "allotment" or "allowance", because the word "data" alone does not refer to the capacity in your mobile service plan. "My cell phone is running out of data" means that the data stored in your phone is not sufficient for your purposes. It does not refer to the data usage allotment you have bought from your network provider. So the really correct thing to say is "My cell phone is running out of data allotment" or "My cell phone is running out of data allowance." In practical life, however, even if you say, "My cell phone is running out of data", people will still understand you. Another correct thing to say is "I am about to go over my data limit." It is about "allotment", "allowance" and "limit". The thing to remember is that a lot of technology street talk is not conducive to good English.

"mobile data" is clearer, but you can say "data" if it's obvious what you're talking about.