I am worried i have breast cancer?

I am 31 and I went to my doctor today as for about a year my right breast has felt heavy and achy and my nipple is very tender,I saw my doctor a few months ago with the same problem and was given antibiotics which never worked.my 34 year old sister was diagnosed 4 days ago with breast cancer which made me go today to the doctor and because it is an ongoing problem and given my sisters diagnosis,she has referred me for a scan on my breast and now I'm terrified I may have breast cancer too,my doctor said she can not feel any lumps that were of any concern-I'm naturally lumpy in my breasts but she could feel no worrying lumps but also as I am a big cup size she was worried she might not feel anything like she would in a smaller woman so she wants me to have the scan and I am really scared of the result,any advice would be great.many thanks