Worried regarding dad's behaviour.

Yes , you must inform to your mother and try your best to understand him knowingly or unknowningly he is doing wrong , it is a cruelty upon your mother, also it attracts dv if he tries or buildup illicit relationship, it is also breach of trust .

But I am worried about mother's reaction after telling her about dad's behaviour. I personally want to talk with dad about it but I am getting fear by how to raise this topic with dad. He is very good person but unable to understand why is he doing like these days. These started just before a week.

it is your duty to make wrong from any one before going thorough it

first talk to your dad and dont not make it a big issue by spoiling your dad image and if he is not changing his behavior  to you then inform your mother or make any close friend of your dad to mediate the issue and solve it for you