Why my CentOS droplet is eating all the RAM memory?

I started with a 2vCPU 2GB CentOS 7 x64 droplet installed from marketplace with a Plesk image. I have only create a Wordpress website with no more than 400 visits per month and followed the initial server setup for CentOS 7 on DigitalOcean.

Then I have started getting alerts from the DigitalOcean monitoring service because the droplet was using more than 70% of memory. I receive those mails 7 times per day, memory usage and resolved ones.

So I thought I was running out of memory, then I have purchased 2GB extra. So my droplet is now a 2vCPU 4GB. After 3 days, same as before, the alert emails for memory beyond 70% started to get my inbox.

At this particular moment, the server is upside that 70% of memory so I ran the command I know DigitalOcean uses to calculate memory (https://www.digitalocean.com/docs/monitoring/resources/glossary-of-terms/#memory)

cat /proc/meminfo

The results are:

MemTotal:	3880364 kB
MemFree:          287080 kB
MemAvailable:    2561780 kB
Buffers:              32 kB
Cached:           994716 kB
SwapCached:            0 kB
Active:          1224116 kB
Inactive:         455532 kB
Active(anon):     769364 kB
Inactive(anon):    27284 kB
Active(file):     454752 kB
Inactive(file):   428248 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                16 kB
Writeback:             0 kB
AnonPages:        684948 kB
Mapped:           101816 kB
Shmem:            111748 kB
Slab:            1750432 kB
SReclaimable:    1678876 kB
SUnreclaim:        71556 kB
KernelStack:        5552 kB
PageTables:        17372 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     1940180 kB
Committed_AS:    3978072 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       95708 kB
VmallocChunk:   34359537660 kB
HardwareCorrupted:     0 kB
AnonHugePages:    344064 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       98276 kB
DirectMap2M:     4096000 kB
DirectMap1G:           0 kB

The command free -m results are:

                     total          used        free      shared buff/cache  available
Mem:           3789         935         240         109        2613        2464
Swap:             0           0           0

Plesk on the other side, trough Grafana is telling me that memory:memory-used:value 858 Mb memory:memory-cached:value 971 Mb memory:memory-slab_recl:value 1.6 Gb

Any ideas? Anything that would help me understand this situation and solve it?