Ditto what Clarabelle said. I used to ALWAYS be told I had fibrocystic breasts by my doctor, every year. I never drank coffee but I used to be addicted to Diet Pepsi. I have now given up ALL caffeine and it has been years and years since I felt a lump, and since my doctor said I had fibrocystic breasts. You know, caffeine is great but it is just not worth the worry.
As for your lump, once I had a terrible lump, it felt like a stone. It was huge. (I was 28 at the time). When I walked down the stairs I could even feel it in my breast, like you'd feel a stone in your shoe. Really! It had shooting pains and was terrible. Turns out I had mastitis. I didn't think I could get that as I have not had kids (I thought that was a breastfeeding problem. ) But that is what it was, so I took two or three weeks of antibiotics. After the infection went away, it took 3 months for the lump to completely dissolve--I went back once a month for 3 months.
Anyway, I'm 33 now and that one hasn't been back. I'd be very surprised if you have C, but of course check it out. I know a lot of people who've had it, and none of them had pain. Also, all of my friends who have had mastitis describe it like you did, and that is what it felt like to me.
Kick the caffeine to the curb, and good luck!!!
Steph, cut out ALL caffeine and see if that helps. My 2 friends and I had these issues and they went away in all 3 of us when we cut out the caffeine. We even joke around now about Pepsi and Caffeine-Free Pepsi calling them Lumpy Pepsi (regular with caffeine) and Non-Lumpy Pepsi (caffeine free). 
