This could just be insecurity and stress. Not understanding things and forgetting things after a conversation is absolutely normal. Also saying you understand when you don't is also normal, it is bad behavior but it is normal bad behavior.

The big question for ADHD is if you can focus, so if you understand something can you do the work? If you can then you should look at the insecurity issues first. But if you have issues with focus even when not talking to people go get checked for ADHD.