Apparently you inserted rows into that table without using the sequence and that's why they are out of sync.
You need to set the correct value for the sequence using
select setval('context_context_id_seq', (select max(context_id) from context));
Then the next call to
nextval() should return the correct value.
If the column is indeed defined as
serial (there is no "auto increment" in Postgres) then you should let Postgres do it's job and never mention it during insers:
insert into context (some_column, some_other_column)
values (42, 'foobar');
will make sure the default value for the context_id column is applied. Alternatively you could use:
insert into context (context_id, some_column, some_other_column)
values (default, 42, 'foobar');