One of the node of cluster in mongo db gets swap free percent down

Hi, I have 3 node cluster of mongo db. On one node from prtg monitoring system I get this notification error:
swap free percent is below the error limit. How to investigate this problem?

Review the swap notes in the MongoDB manual:

If MongoDB is the only function the server provides then it could just be tuning of vm.swapiness. If this server is multi-duty it could be other applications that are swapping out.