ok, you don't want to tell them, but "PERSONAL REASONS" is such a crappy excuse. It culd mean anything from my partner is dying of aids to i couldn't be bothered to get out of bed. If it was the former, they should give you some leeway, the latter and they wont, but how are they supposed to know which end of the spectrum your "personal reasons" are?
I dont know what your terms and conditions of employment are, but mine includes a paragraph about who to contact if you are unable to attend for your work and it says something like cntacting them before your shift starts. If yours is the same, then they have given you fair warning of what to expect, and like i say they might give some leeway, if only they knew what the problem was