Can't match my last cardio session?

On Friday I did a seemingly easy session of 30min on the stationary bike and have been doing it pretty consecutively for the past 3 weeks or so. I try to do 30min sessions 5 or so times a week mixed with weights in between.

However, I noticed that some days I can do 30mins and feel like doing more however some other days I am pushing 15mins and then split the last 15mins up into 5 minute segments. It's not a matter of running out of breath either. My legs get very tired, I can begin to feel the weight of the bike.

I'm 5'9 170lbs male and I am confused. I'm doing the exact same bike configuration each time and mostly work out around the same time.

Could this be down to my eating? I noticed that on Friday I had dominos pizza and matched my caloric goal of around 2000 calories. Where-as today I have been pretty clean, ate my Sunday roast and a sandwich on later on and am at 1400 calories with my macro goal met also.. i'm a little confused on what's happening.