AWS High Memory Usage. How to detect the cause?

I'm using Amazon EC2 instance t2.small. And the memory usage is always increasing up to > 90% usage.

Memory usage > 90 %

$ free -m
             total       used       free     shared    buffers     cached
Mem:          1993       1925         68          0          1         24
+ buffers/cache:       1899         94
Swap:         2047        994       1053

This is my memory usage :

$ps aux --sort=-resident|head -11

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
webapp    8640  9.4 81.1 3223112 1657264 ?     Sl   Oct24  28:00 puma: cluster worker 0: 9938 [current]
root      9862  0.1  6.7 1057364 137712 ?      Sl   Oct23   2:50 sidekiq 4.2.10 current [0 of 5 busy]
healthd   2784  0.0  0.9 684904 19360 ?        Ssl  Oct18   4:25 puma 2.11.1 (tcp://127.0.0.1:22221) [healthd]
root      4057  0.3  0.6 536908 12652 ?        Ssl  Oct18  29:52 /usr/bin/python2.7 /opt/aws/bin/cfn-hup
nginx     9905  2.6  0.4 156088  9748 ?        S    Oct23  67:02 nginx: worker process
webapp    9938  0.0  0.1 121660  3800 ?        Ssl  Oct23   0:05 puma 3.1.0 (unix:///var/run/puma/my_app.sock) [current] 
ec2-user 21670  0.0  0.1 117372  2544 pts/0    R+   02:39   0:00 ps aux --sort=-resident
root      2309  0.0  0.1 276100  2140 ?        Ssl  Oct18   0:10 /usr/bin/amazon-ssm-agent 
ec2-user 15697  0.0  0.1 115488  2124 pts/0    Ss   00:27   0:00 -bash
root         1  0.0  0.0  19692  1800 ?        Ss   Oct18   0:01 /sbin/init

I can't find out why the webapp memory usage so high and it's always increasing. So for now we set a job to restart the instance once in a hour. Is there any way I can figure out what causing high memory usage of my webapp? Thank you so much.