I am 25 and have Type I diabetes since I was 6. No history of breast cancer in either sides of my family.
I recently (~one month ago) started to have a mild pain in my left breast, also "irradiating" towards the armpit and the back. I checked for lumps/ swells, but I find it really hard to tell if there are any, especially because I have small breasts, so it's difficult to say whether there is some extra tissue apart from the normal one. However, there does not seem to be a significant difference between the size or shape of the two breasts.
I am also on Yasmin BCP (since 2 years ago), but I still have an irregular menstrual cycle (probably due to diabetes and some ovary cysts). The pain does not seem related to the cycle.
I do not have a regular sexual life - I only see my guy once a few months. Actually, while I saw him the last time, the pain seemed to cease.
Frankly speaking, do you think my paranoia is at all justified? Do you have any suggestions for how I should deal with it?

Any time you have chest pain of any kind, whether or not it is in the breast, you need to seek medical attention.

It is doubtful that this is breast cancer, but there may be some other issues that should be addressed by your medical professional.