Sore boob...unnecessarily worrying?

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I have a sore boob. I'm not breastfeeding (DS is 4), and I couldn't anyway when DS was born so it's nothing to do with that.
One boob has got really heavy, and sort of feels like it is 'pulling' inside and is really tender. It's also bigger than usual.
Am I right to worry? My sister has breast cancer when she was three years older than I am now.
Somebody offer some words of wisom.

I'd definitely go to the GP just to set your mind at rest. It's what they're there for.

I have an appointment a week on Monday (about something else non-emergency) but I was wondering if I should try to get in earlier.