Normal. (Don't let so-called "experts" give you a psychiatric diagnosis for behavior well within the normal range in later life, either...)
Check out those child development books every time you start seeing new behaviors... Around now (or maybe he's a little early? I forgot, exactly) - he is discovering that he HAS a will of his own. It's not exactly a fun period, from this respect...but - when he learns, around age 18 or so, that he can go out on his own, become responsible for his own life, pay his own way, etc...(well - that is painful for other reasons...but - it IS a proud period, too...seeing him become a successful father and husband, etc, etc...)
But - this step along the way is necessary...You can't talk him out of being difficult, you MAY be able to distract him with something he DOES want to do while you take care of business...I used to give them something I normally wouldn't let them play with - not dangerous, of course, but something that required my close supervision while they had it, they were curious enough to stop fighting for a moment, etc. It was not easy to avoid a battle of wills...but - as best you can - for all such battles (and there will be many :-) try to make the solution a win/win and not a "my way of the highway" situation...and save the "my way of the highway" for "don't drink and drive" and "don't do drugs" sort of issues... I'm not saying give in and let him win and you lose... I'm saying find a way so everybody wins whenever possible... He will learn early that that is how you work things and will be much more likely to let you know when he feels like he is losing, will be much more likely to entertain you getting your needs met when there are disagreements about how things are going to happen, etc, etc... (see: it's normal - and now sets the tone - and - sometimes, for now, he'll just be angry - and - without upset, make sure he sees - you don't have to tell him, he will see - that, ultimately, you decide what is best for him right now - and - eventually those diapers HAVE to be changed! :-)
Enjoy!