which process has created a load spike on my server last week?
If you have the slow query log active, you might be able to identify a long running query during the timeframe of your load spike. This probably the only way to figure out the issue that has already occurred.

You can configure a script like pt-stalk to try to capture future issues, however. 

Here's an example to trigger data collection if you have more than 10 connections from host '10.0.0.23' [src]:

pt-stalk --function processlist --variable Host\
 --match 10.0.0.23 --threshold 10 -- --defaults-file=./my.default.cnf