Anastasia,

The neck issue is a simple answer. Since most concussions are the primary concern, the neck issues are overlooked. But, the head is critically supported by the neck so any trauma to the head will include some level of trauma to the neck. Since the neck issues are not easily treated by medical doctors, they do not check the neck unless there is a significant complaint.

A common problem with concussion is concussion whiplash or a similar injury. The head aches can be from the concussion or from the neck injury that can result in neck muscle spasms up into the head. There are neck muscles that attach to the skull behind the ears. When these spams, they often manifest as a headache. They can also cause a true head ache due to the effect they have on a myriad of systems related to the head (circulation, muscles, and sleep, etc).

If your exercise bike works at 105 BPM, you will likely get good results. Bike exercise causes much more blood flow that other exercises because the legs act like an addition blood pump. It takes serious oxygen debt to get pulse rates up high on a bike. Such an effort may be too much for your brain. Besides, you do not want oxygen debt when exercising for your brain's sake.

Have you taken your blood pressure while doing or right after your 20 minute session? It will help you understand what strains you are putting on your brain.