Here is my take:
1) The 11-13 yo range of age romances- Not that new. I had a little boyfriend when I was 12. It didn't go beyond kissing and holding hands, but we had classes together, went roller skating on Friday nights and sat together on the bus to away games. It lasted the length of basketball/cheerleading season and then it was over. But there were many a discussion about who was sleeping with who and who was cheating on who...in 7th grade. This was about 1984. Do I think that pre-teen should be having sex? No, not really. But I don't see any real harm in a more innocent, well chaperoned romance in junior high.
2) I don't think it is ever too early to talk to kids about sex, relationships, and dating and to instill your values into that discussion. Let her know what you think about it and why you have concerns. Be honest with her. She may listen, and she may not. She may be telling you because she wants you to set the boundaries...let you be the "bad guy" to save face, even if she isn't conciousy aware of it ("My mom says I can't date...She sucks...Bye Bye now"). Or she may be having strong feelings and really needs someone to talk to and sort it all out with her about what is "normal". Or it might be of a more innocent nature and she is unaware that you might have concerns. It is hard to know unless you have an open and frank conversation with her about it.
3) Unless you think she is actively engaging in unhealthy practices/putting herself in danger, I probably would not break her confidence. If you think she is putting herself in some kind of danger, than I think you have a responsibility to tell her father. Just know you risk alienating her and potentially endangering her trust in you, something that might take a long time to re-build. And if you are going to tell her father, I would be honest with her about your intentions and why you feel like you have to tell him. Maybe you could all have a joint conversation, with them on "Speaker phone" on one end and you on the other end. It isn't the optimal arrangement, but it would hopefully get all three of you on the same page and to a place of understanding.
Mama to three small people; wife to one big person; pet-person to cats and dogs..."Be the change you want to see in the world"-- Gandhi