Lumps in the breasts happen alot in pregnancy. They're swollen glands usually, but they can sometimes be cysts. Sorry to hear about the possible miscarrage. :-(
I am sorry for what you are going through. However,
1. dont give up on your pregnancy, you may not be miscarrying
2. dont ignore a lump in your breast because it "may" be from being pregnant
Any time your breasts feel different to you or your doctor, you need to have it checked. Mammos are not 100% so always do self checks. You know better than anyone else how your breast feel. Good luck.
You definitely need to have the lump checked but........I wouldn't worry too much. My best friend got a lump in her breast right before she took her hpt to find a BFP! We have a mutual friend that is an OB/GYN nurse and that woman told my friend to take the test because she knew that lumps are common.