Google costochondritis.

Get it checked out.
I am the same age as you and had this a couple of months ago. It lasted about 6 weeks.
I went to dr as soon as pain started and had mammo, sono, all negative. Pain was still there so saw breast surgeon who thought it was hormonal. I think she was right. Pain went away.
It is typical of the age but do go and have it checked please please please.

Please get it checked out. No one will think youre over reacting.

I agree that an appointment with your OB would ease your mind. I was once sent for a mammogram and ultrasound for an area that felt thicker than surrounding tissue. It turned out to be nothing, but then I could stop worrying about it.

Go ahead and make an apt. with your OB who will refer you most likely for a diagnostic ultrasound or breast surgeon. I have experience in this area (unfortunately) and it is better to go through more testing than let things go too long without answers.

I'm 43, and had my first mammogram last year, no issues, no family history.

The last few days I feel a kind of tender, maybe swollen/slightly harder/denser area on one breast, but it doesn't feel like a hard lump. I've also been lifting weights a lot more, and perhaps it is strain, since its where the pecs come in. It almost feels like it did when I would nurse and start to get a hard area from build up.

this doesn't seem like the classic lump, but wondering whether I should give it a few days to see if its muscle or hormonal or make an appt.