Human behavior can easily be learned with time. As you spend more time together, you get to know each other better and pick up certain patterns. When a person changes, they will show it through their actions and your gut feeling. If your gut tells you he has changed then no doubt he has changed unless you are having a bad stomach of course. This is why your man might start acting strangely:
He has lost interest in you
When a man loses affection for a woman, it is very difficult to hide. It will start with a reduction in communication, then avoiding spending time together, less responsibility then comes to the numerous excuses and eventual breakup.
He has fallen for someone else
Men are very poor at multi-tasking and we all know it. If a guy falls for another lady, there will be an obvious indicator that his heart is somewhere else. a keen lady will notice when his man's heart is being taken from her to someone else or worse still is being divided and a portion is given to another catch.
You have become unmanageable
Sometimes a man might act weird because you are pushing him past the breaking point. You might have been overdependent asking him for all manner of things ranging from airtime, pocket money, rent, salon bills, shopping and such. You might as well have picked up some unbearable manners like being too nagging, controlling, disrespectful and lying.