I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
If you are a non exempt employee (which means you get paid hourly), any overtime you make is separate and apart from sick and vacation day issues. The overtime you make is extra earned income because you worked in excess of the "regular work week hours". Regular work week hours vary between employers -- some are as little as 36, some are 50 and of course anywhere in between.
That overtime is yours and if they attempt to take it away from you by trying to deduct from your sick days or vacation days, that amounts to theft!
Contact an employment labor law attorney who may be able to get this matter resolved quietly and quickly.
Try attorneypages.com in your state. If you can't find one, call (dial 411) your STATE BAR ASSOCIATION and ask for lawyer referral programs.
Hope this helps.