[GENERAL] Service outage: each postgres process use the exact amount of theconfigured work_mem

Hi all,
About 2 hours and half ago, suddenly (and on the late afternoon of the Easter Friday), customers reported failing connections to our server, or even very slow.
After a bit of checking (that also involved server reboot) I noticed (using top) that every process regarding postgres is using exactly the amout I configured as work_mem (3 GB). And AFAIK it's not good.

30085 postgres 20 0 3370048 156656 153876 S 6.7 0.3 0:00.20 postgres
29833 postgres 20 0 3370000 65260 62416 S 1.7 0.1 0:00.17 postgres
29632 postgres 20 0 3372468 11712 6028 S 0.7 0.0 0:00.60 postgres

What can be happened?
Nothing has been touched....
postgresql 9.5.6 on debian 8 just apt-get upgrade'd

Any help would be appreciated.
Moreno.