You might have flown too fast: this sometimes causes the dream to blur and destabilize. Or, it might have simply been excitement, in which case this will go away eventually. You'll still experience a jolt of excitement upon lucidity (I still do!) but it won't be enough to wake you up. Another possible reason is you got into your dream activities too quickly after becoming lucid.

Optim's linked a good tutorial so be sure to check it out too.