What should I do with someone who takes a while to reply?

I have seen my gf always getting online in Instagram , but she has not replied my msg for a long time, around 3-4 hours. When she takes a while to reply, I would overthink that she doesn’t like me or she is not interested. Also, when we are outside talking face to face, I feel it is different from talking online. Like talking online , I feel like it lacks enthusiasm cuz her replies seems short, but talking outside face to face is like the opposite. So I want to ask that is there any problems with me or is it me over-thinking?