this is a strange (i think) thing...
different 'teachers' and also certain books 'teach' you skills you may use. the problem arises when the teacher or book presents a lower level of instruction than you already have! you revert to the 'new' level, even if it should be 2-8 levels below where you were. ouch. <img src="/ubbthreads/images/graemlins/puppyeyes.gif" alt="" />
it sounds as though this is what's happened to you.
my suggestion is this - always have a notepad and pen around your computer. list every skill and level of skill you acquire. NEVER read a skill book withour saving prior.
personally, i also have a 'stat' sheet for all items worn (and carried) for each of my characters too. this way i can refer to the sheet when visiting a merchant - keep in mind that whatever is worn by your non-leader is not visible while with merchants.
kiya (or someone else) will probably help with exactly where the skill can be purchased (raised) again.
i hope this explains it.