Is it true that babies always cry for a reason?

I don't think this can be true because my 3 and half month old baby girl has been crying and fussing all day for no reason at all. I did all the basics like change her diaper, feed her, burp her, and make sure she is clean. I tried singing to her, rocking her, talking to her, playing with her, just letting her be in her swing because I thought maybe I was overstimulating her. I just can't think of anything else to do. I am at my witts end! She just keeps crying! Her temperature is normal, she doesn't seem sick. She doesn't have a stuffy nose or anything like that. Her diapers are normal. I walked around with her in my arms but no, she is not happy. I would have taken her for a stroller walk today but it's been pretty chilly. Am I doing something wrong? I am trying to make her as happy and comfy as possible. People say that babies cry for a reason but I have no idea what she could be crying about.