Hi, and thanks for your medical question on eHealth forum!
If there is no change in the duration and flow of periods, then it is unlikely that you are pregnant. However, since you are having breast changes, take a blood pregnancy test to be 100% sure.
If this is negative, then breast darkening and tummy enlargement can be due to PCOD, adrenal gland problems, thyroid problems and hormonal imbalance.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with your gynecologist and get this investigated. Hope this helps. Take care!