can i buy more data just for this month

You didn't say what plan you were on.  Whichever plan you are on may make a big difference on whether you are done for the month, whether you will be charged overage charges, or whether you don't understand your unlimited data plan.

The name of your plan is listed in your online account on the overview page.

Some plans charge you overage charges for extra data. So in a sense you are paying for more data. Other plans are deprioritized after an individual line uses up all of its high-speed data.

And there are a couple of grandfathered plans which were capped at a fixed amount of data.

Based on the way you worded your question, it sounds like you have an unlimited data plan and have been deprioritized because all of one person's data has been used up. Unlimited data plans are per line, not per account.

So if you are on an unlimited data plan and your plan depriortizes at 22 GB, only the person who has reached 22 gigabytes will experience deprioritization, not everyone 

Can't say for sure without plan name.

If you are on the mobile share advantage plan, and I think the mobile share flex plan, both capped data at a hard stop. If you are on one of those plans, you cannot purchase extra data. You're only way to get more data is to change your plan to one of the current unlimited data plans