Why my dog pooh looks like skinny and few small drop shapes?

few days ago my dog started to pooh looks like skinny and few small drop and his tail is down most of time and he sometimes whines urge to go right away. i took him to bathroom more often then before. yesterday i took him to bathroom go pooh 5 or 6 times because he feels like he wanted to go but then he didn't pooh but i can tell his trying. he pooh very well before and go regular time. i am not sure what cause to do this,