You may not be tolerating the commute mentally. Subways and trains have a lot of stimulation. Add that to anxiety and you get the perfect storm.

Even if the bump was a 'sub-concussive' bump, that is almost meaningless. Sub-concussive impacts are cumulative. But, one needs to accumulated hundreds of them to cause injury.

The surface pain of the bump can trigger a strong response. The startle of the pain plus the history of anxiety about head impacts can be a substantial trigger.

Regarding sleep. Quantity of sleep has no direct bearing on quality of sleep. In fact, many who sleep longer hours do so because they are not getting quality sleep. Quality sleep means you are experiencing all of the stages of sleep. Struggles with memory are often related to lack of REM sleep. If one is getting quality sleep, one would usually wake up refreshed with no desire to snooze.

Regarding studying for your license exams, It is best to space out your study. Review the information over time. Memorizing happens during REM sleep as short term memory is converted to long term memory. So, rather that repeat the same material over and over during the same study session, review it once each session then repeat that session on a subsequent day.

If you double up, try a study session during each subway ride. But, wear ear plugs or headphones so your brain is not distracted with ambient subway sounds. Study without an ability to stay focused is of minimal value. There has been extensive research that shows that cram studying for an exam is not as effective as spreading out the study.

When you are talking with friends, try to limit the number of voices speaking at the same time. It will only mess up your ability to function in both speaking and remembering. Your brain cannot multi-task. Even a healthy brain struggles to multitask.

Try to spread out the different exams so you are not doubling up on your studies.

The toughest part of PCS is learning the new ways we need to learn. The old ways of learning often do not work well at all.

So, take you time. Destress your life and give yourself time to learn and you will be fine.