Bulbs Breaking?

[AkHerps]

I think I'm buying a new heat bulb about once a month right now, and that can't be right.

I buy a 100 watt heat bulb from Petco (our only pet store), put it in his ceramic light fixture (which is rated for over 100watts), which is on a lamp dimmer, and then plugged into a timer.

This morning I woke up to another blown bulb, when I just got this one last month.

They aren't breaking, just burning out.

Any ideas as to why this keeps happening? His lamp and bulb are zoo med.

Thank you!
Sara