Why do my boobs ache badly during my period?

I’m 25 so not new to the whole ‘having a period thing “, but what’s new for me is how badly my boobs are aching. I couldn’t bring myself to wear a bra today. I’m on day 2 of a light flow which is also weird for me considering I’m not on any contraception. Until this bleed I was wondering if I was pregnant because of how I’ve been feeling. Only thing is I’ve had no menstrual cramps either. What a weird month December is. 🤷‍♀️