Lump in boob?? Helppp

I have this lump like thing on my boob, on the underside. And it’s been there for probably about a month. But it started out kinda small so I thought it was maybe like a pimple or something but it’s like in my boob, and there’s no white head or anything like that, and it’s gotten a lot bigger. It’s sore but not like crazy painful. But it’s pretty big and it’s scary to me. It’s not squishy, so I don’t think it’s like any type of pimple or anything like that, it’s kinda hard? Idk. Idk what to compare it to, I’ve never had anything like this. I’m going to call my dr tomorrow. I just wanted to know if anyone has had a similar experience??