It is most probably due to hormones --- unless you have some other cause for breast tenderness such as starting to work out, a severe respiratory infection (chest wall inflammation), etc.
You mentioned that you did not have your usual breast soreness before this period. It arrived after the flow. My best GUESS is that your last "period" may not have been linked to an ovulation. Classically, periods caused by ovulations have the most prominent premenstrual symptoms (breast pain, cramps, etc). Also, if you missed an ovulation last month you have been exposed to continued levels of estrogen. Estrogen has been a purported cause of breast tenderness and enlargement.
Hopefully, this is just a temporary thing. If the bilateral breast pain persists or you develop other symptoms such as the pain and blistering of shingles on the chest, you should see your GYN or clinic for a breast exam.
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