I am on probation in fl for a misdemeanor drug possession charge. While on probation I have just received a dui. What should I do? What should I expect? Should I tell my probation officer now?

Yes you should tell your PO rather then have them find out themselves. What will happen depends on the terms of your probation.

Yes you should tell your PO, they will find it when they do a computer check on you from time to time, and hearing it from first means you are being responsible.

Was the DUI drugs or drinking?

If for drugs, it will most likely put you on a closer probation level, if drinking, most likely a lecture ( but this is my guess only)