Can diet pills cause your breasts to be sore?

Yes: Hydroxycut does cause breast soreness.

Possible.: Caffeine has previously been associated with breast pain, but randomised trials have not found a link. Still it seems that some women are affected by caffeine. Many diet pills contain caffeine like substances so there may be a link.

Possibly?: It depends on the actual ingredients contained in the pill. My best advice is to stop them for 2-3 weeks and see if the soreness goes away. On the other hand, could you be pregnant...?