Elastic Stack memory pressure increase over time

Hello,

Last month I have upgraded my elastic stack from 8GB Ram to 16GB since I had high memory pressure. It all worked out until last week when memory and CPU started to increase day after day without any change or increase in the data I'm ingesting/indexing. Today, memory pressure was above 80% so I restarted my instances and it got down to 30% after the restart. I am trying to drill down to the issue but I can't find a reasonable explanation to my problem and I am afraid that it might gonna happen again. What thing should I search and check to understand why this is happening?

How many shards, how many nodes, what version?

There are 2 Elasticsearch nodes running on 7.9.
Indices and shards:

      "indices" : {
        "count" : 167,
        "shards" : {
          "total" : 334,
          "primaries" : 167,
          "replication" : 1.0,
          "index" : {
            "shards" : {
              "min" : 2,
              "max" : 2,
              "avg" : 2.0
    }