Chems crashing.....HELP!

I checked my chemical levels this morning and bromine, pH, and alkalinity had all crashed to really low levels! I am new to hot tubbing, but have never had this happen before. I follow a pretty standard chemical regimen. I haven't been in my tub for about 2 weeks, but everything was fine about 5 days ago when I checked it. Any idea what happened and how I fix it? How do I make sure this doesn't happen again.