Amazon RDS low in memory no mysql connections
The memory will be consumed by a number of things, including the MySQL innodb buffer pool, table and definition caches, etc. 

RDS will default the innodb_buffer_pool_size to {DBInstanceClassMemory*3/4}, which is a good starting point, but does only leave a 1/4 of the RAM available for other parts of the system.

One thing that really hits the small RDS instances is the performance_schema - this can consume 500Mb RAM as soon as the server starts up, and never releases it.

If you are not using the performance_schema regularly, if at all, turn it off by setting the appropriate parameter within your RDS parameter group.

You will need to reboot the RDS instance if this is the case.

At the end of the day, a t2.medium instance only has 4Gb of RAM available to you (see http://www.ec2instances.info/rds/), and 52Mb is probably more than enough free for your system to continue to work without issue. Most of your data content will be in the buffer pool, so MySQL will respond directly from memory for most of your queries, and feel pretty responsive, even on such a small system.