As Rowuk states, this could be a problem with anything from breathing to articulation.
I'd do the following:
- work on breathing
- review and work on your whole support column (diaphragm, breath, body position, throat, articulation, embouchure, etc.) and make sure things are 'right'
- don't overdo your warmup! there's a difference between warming up and practicing. Your warmup should just get you ready to practice
- go back to fundamentals. Get out the Arban's book and practice some of those exercises you forgot about years agao
- make sure you're not dehydrated - drink plenty of water