As someone who has stage 4 cancer at 44, most cancers have no symptoms or very subtle symptoms until it's too late and it's spread.
Doctors often overlook any small symptoms particularly if you do not fit a cancer profile: You are young, you don't smoke, you have no risk factors. And, the small symptoms you do have are easily explained away, because they are symptoms of many other less significant issues.
Mine was overlooked because I was young, female, and fit. It's usually an old smoking man cancer. Well, surprise!
It's hard to understand until you are in it. The human body is weird. When we get a cold, you have immediate symptoms, are stuffed up, etc. and it's a nothing illness in the grand scheme. But when our life is actually threatened and it's serious, like cancer, there are no signs until it's too late.