To ask the simple question, Even though your system has 16GB of ram this does not matter to a JVM. Elasticsearch by default runs the JVM at 1GB.
https://www.elastic.co/guide/en/elasticsearch/guide/current/heap-sizing.html
export ES_HEAP_SIZE=10g
Also, you may want to look at the type of searches your doing and the Number of them. If you are hitting OOM like Warkolm mentioned you might have to add more nodes.
Though you could look at improving your searches to and indexes so that they are not retrieving too much data, and having to search to many indexes.
Just some more idea's