Progress Question

I started going to the gym regularly about 5 months ago. I have already noticed much progress, especially in my bench. My max when I started was 95lbs for 8 reps. I have since increased that to 135lbs for 8 reps. My problem is that in the past 2 weeks, I could barely squeeze out 4 reps of 135. Is this a training problem, or something else? I have a set routine, and I haven't been lacking when it comes to going to the gym.