Period pains (cramps/sore breast) between periods can be a sign of many things. You need to see a doctor and explain exact pain and location. It could simply be a cyst which may go away on its own; or a more serious cyst which needs surgery. It could also be IBS or even pregnancy. An ultrasound will show up any cysts or abnormalities. Do you have other symptoms like pain when you go to the toilet? This would establish if it is urinary or GU related. It also could simply be an infection which can be treated easily... The doctor is your first port of call.
