Only my nipple hurts?

I’m not sure why this is happening.. I’m on combination birth control, almost a year now, plus me and my boyfriend use condoms so I’m pretty sure I can rule out pregnancy? I had a period in May and had a light period in June but it was dark brown/old blood with some clotting. I have not had sex since my last period since I am studying abroad.

Why could this be happening? I have another 2 weeks until my placebo week. I tend to worry a little about pregnancy even though I am very safe so I kept touching my boob to make sure it wasn’t growing or sore so maybe that’s the cause? If not, what could the cause of pain on the nipple be?