Using books is not uncommon in Alexander technique because they are both firm and (as you said) the level of your head can be adjusted easily by adding or subtracting one. It is probably that because you have grown used to being misaligned your muscles that haven't been used properly in a while are being taxed. It should go away after a while. If it's too much, let her know when you go back and she should adjust her exercises. Feldenkrais is another good option as is somatic movement if you have that available to you. I haven't done Feldenkrais personally but know of it and know those who swear by it. Somatic movement I coach with.

Did she give you any mantra to practice with at home?