I personally wouldn't go to the doctor at this point, I think most women have had times in their lives when an odd pain appears in one breast or the other. Firstly you can find nothing that is concerning from your manual examination (presuming you did the correct manual examination; i.e. lying down not using fingers tips etc etc). Breast tissue is like any other tissue, it can get bruised internally, have nerves that play up, get hurt from bangs or ill-fitting bras or lying on your breast in an awkward way, or react to hormonal changes. Pain when pressed, as a symptom on its own, is highly hugely exceedingly unlikely to be anything concerning. If I was you, I would monitor for a few weeks or months. BY 'monitor' I don't mean press it every few hours and prod around lol, I mean leave it well alone and maybe have a look each week at the breast shape in general and the specific area. Pressing and prodding by itself can cause inflammation and bruising. Even IF there is something that is causing the pain internally, then benign causes are the most likely reason anyway, especially for a 31 year old!. If you notice changes over the next few weeks and months, then of course go visit the GP.