Got a concussion a month ago, fell on my bum yesterday, any chance of further injury to head?

Not likely: It does not sound likely that there was sufficient mechanism with the injury you are describing to have caused a concussion, but certainly if you are having head aches, changes in your vision or mental function, I would be checked.

Unlikely issue: If you did not strike your head, and you have no head symptoms such as headaches or dizziness, would appear that you did not experience further head trauma.