firstly, not a chance these gains are going to keep coming. i u were on tht extreme of a cut firstly, it is completely unneccesary to have THAT low calories and carbs no matter what, so ur body going from 100g to 340g of carbs if a huge jump, ur muscles will have ALOT more glycogen and ATP at their disposal. creatine also helps in strength gains so i would say it is a compination of both.
when i started gaining last summer i had the same respone, i went up about 10kg in bench in 3 weeks. it doesnt last unfortunately :P enjoy it while it does! they will be the best gains of your life when u start bulking for the first time.