How can I find out who just assigned me a collaboration task?

Good question! Unfortunately this information currently isn’t included in the email notification you receive when the task is assigned.

The quickest way would be to click the View Item link in that email notification. That will open the Info page for the corresponding process item, and if you scroll down to the Task History table, you'll see who else has an In Progress task, and is likely the one that assigned you the collaboration task.

However, if you need to find out definitively who assigned a particular task, you need to go one step further, into the audit trail for the task. From the Task History, click the Info link for your collaboration task,

and then click the View Audit Trail link. Then you'll see the change history for that task, including who first created it.