You should see your health care provider for some tests. It is most likely due to a hormone fluctuation BUT it could potentially be something more serious (for example, very rarely this will happen as an early sign of breast cancer). It is particularly worrisome if only one breast is affected.
Get this checked. Besides cancer, some medications can cause the hormonal shifts. I had it happen once and my breasts started producing a bit of milk discharge. Uhhh....I've never been pregnant in my life. If it is cancer, it is vital to find out in the early stages.
Thank you :) so I should be scared if it is only one breast?