Found a lump?

I'm going to try to explain this in the best way I can. 
Okay, so, I'm 19 years old. I'll be 20 in July. My breasts are pretty small, an A-Cup on a good day. This weekend I noticed that my breasts were kind of sore. I start my period in about a week, so I chalked it up to that. 
This morning I took a shower and found a lump in my left breast. It is near the bottom. It is very hard and moves around pretty easily. The lump itself is not super painful, but I noticed that if water hit my nipple it literally felt like an electric shock. 
My questions are: 
•Has anyone else experienced this? 
•If so, what was it?