I would say that this sort of 'inconsistency' in your brain's ability to handle situations is perfectly normal for PCS. It's also one of the reasons other people find it so hard to understand and accept what we are going through.

For me I think the amount of sleep I have been getting during any given period is a major cause of such inconsistency. At the moment, if I have been sleeping well I can forget I ever had a concussion, if I have not slept well it affects me much more than other people and the sort of social activities you mention become a challenge.