Why was it hard to start urinating one night ?

I am 32, very fit, dont drink, don't smoke...one sex partner for the last 15 years... 3 nights ago I drank a big glass of milk before bed, and got up about 4am to pee, It took me about 30 seconds to start peeing....I was a little groggy, but don't recall ever having this before. My urine was cystal clear, I have no pain...and the next time I went my urine stream started right away like always, and haven't had any issues since. What could have caused this? I realize that enlarged prostate is the most logical choice, but I would think that would happen more than once....any ideas? Could it have been nothing? perhaps bladder muscle related?