I would think the lines of communication have been wide open all along so she feels ok to come to you to talk and not be judged or "banned" from doing sometihng. When you talk to her, have an open mind and just listen..... don't say much.
She is 11, and in the midst of body changes, etc and it is scary. BF's are not like they are for 11 yr olds vs 17 yr olds. It is a passing fancy to have a BF but most of the time, all they do is text or sometimes meet in groups for the movies (although 11 is a bit young in my personal opinion for the group things). I allowed my daughter to go on group outings around 13
We encouraged all of daughter's friends to hang out at our house. I lvoe that aspect of having my home where they can hang out, play pool, Wii, etc and I know what they are doing vs some parents I've been made aware of that just leave them alone and try to be friends vs parents.
I would be concerned about her grades. Ask her why they are down, is something else going on? Communicate with the teachers to get a feel for how she is doing in class as far as participation, getting along with other, etc.
When parents'ban" BF's and certain friends, the child will certainly be more attacted to the activity that is banned. As her about what her "relationship" is like. It is probably nothing more than sitting together at lunch, letters, etc.
Keep your lines of communication open so she know you have her back no matter what and she can tell you anything without you getting mad. I know that is hard to do.... I just went through it with my 17 yr old calling me up on the phone on the way home crying and telling me she just got pulled over for speeding. I thanked her for the honestly (you don't realize how many kids just hide it and it makes it 10x worse) and made sure she knew we would work through it together, even though I had to revert back to carpool line while her car keys were taken for a month.
Hang in there mom, you will do well. This is the beginning of a long, sometimes difficult process but very rewarding as well.