How to rotate a camera?
Promoting my comment into an answer since it identified the problem...

The issue is confusion between degrees and radians for the angle parameter. GLM, up to version 0.9.5, was very inconsistent in how it dealt with angles, accepting degrees for some functions and radians for others. This behaviour could be overridden with #define GLM_FORCE_RADIANS before including the header.

In version 0.9.6, radians everywhere became the default. Unfortunately, if you Google 'glm rotate' at the time of writing, the top result is the 0.9.4 doc page, which still describes the parameters as taking degrees, so you need to be aware of which version you are using and its default behaviour in this regard. If you still want to use degrees for your angle variables, you can convert them with glm::radians(angle) before passing them as arguments.