First off, hourly != non-exempt. Just wanted to make sure that was out there. You can be a non-exempt salaried employee, still entitled to overtime.
Now for your situation, if your job duties have not changed, you should not have been moved to exempt status. The DOL has 5 exemptions:
1. EXECUTIVE (examples: chief executive officer, controller, vice president, director)
2. ADMINISTRATIVE (examples: manager, supervisor, administrator)
3. PROFESSIONAL: LEARNED AND CREATIVE (examples: accountant, nurse, engineer, composer, singer, graphic designer)
4. COMPUTER-RELATED (examples: network or database analyst, developer, programmer, software engineer)
5. OUTSIDE SALES (examples: salespersons, contract negotiators)
Now this isn't the end all be all of the FLSA, but it's the most common exemptions you run across. The DOL's site also lists some qualifications for each exemption, I suggest you check it out (
www.dol.gov).
If you should be classified as a non-exempt employee, I suggest you go to HR and tell them that you looked into the FLSA and you believe your position is non-exempt and that you are entitled to any overtime you worked. If they don't budge, I suggest you call your state's labor board to make a claim.
Basically, unless they gave you a new position before you went out of town (which should have been accompanied by a written notice anyway) then they can't really just switch you to exempt because they feel like it.
Good luck.