Why suddenly I can lift heavier weights?

I went to gym straight for 1 months after 6 months of break. After 1 month I due to vacation i missed 2 weeks. And after i came back fron vacation, i was suddey able to life heavier than i was 2 weeks before. Eg I did 6 pullups max, and today i was able to do 10 without failure. Same for lats, and biceps. What might have happened here? Can anybody explain? I took 30g whey before going to gym, is it becaus of that? I dont think so coz i also used to take whey previously.