That was probably just their way to try to comfort you and they didn't mean to appear to dismiss your worry. I'm sure they don't understand what to do in the situation.
Probably many of us come here seeking understanding from people that also are struggling through our issues. You're right, we do understand. Before you share with anyone make sure they are what I call 'safe' people. meaning people that demonstrate ability to empathize and understand without trying to tell us what to feel. I only know a couple of people who know how to be there for me. and I think it's a learned thing.
By the way, I don't think whatever you feel is over-reacting. We feel what we feel.The thing is we can learn to handle what we feel, let the feelings wash over us and let them fade away. It even helps to look in a mirror and give yourself understanding, and tell yourself it is normal to be afraid of the possibility of cancer, and tell yourself that you are going to take care of yourself, too, and to not worry. Tell yourself that you can relax while you wait to find out what you have to deal with.
Be sure to let us know what you find out. OK?
We are here for you.