Exhausted Half Way Through Workout!

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Recently, I've been getting half way through my workout (usually right after weights are done) and I'm about to start my cardio I typically love cardio so I know its not just disinterest. But I would previously have no issue getting through an hour workout which consists of core work, stability ball work, weights then cardio but the past couple days I've felt pretty whooped by the time I am finished lifting. Any ideas what this could be from? (I've been eating more than I was to begin with since I started adding the weight in.)

And am I the only one that this happens to?