I had sex during my last menstural cycle & my breasts have been tender since, what could cause this?

Breast Pain: Some common causes of breast pain include but are not limited to pregnancy, hormonal changes during ones menstrual cycle, caffeine, trauma, fibrocystic changes. You should consult your physician he may consider some testing such as sonogram or mammogram.

Minor trauma!: If there was any breast manipulation during sex, your already-tender and slightly swollen breasts may have had minor bruising, causing the symptoms you now note. I see this during breast surgery on my patients having or just around their cycles--more pain, bruising, inflammation, etc. So any activity (such as sex) that can cause minor breast trauma can end up being more symptomatic than usual!