S.,
To ask if you are overreacting is to make judgement on your feelings. This is your first time through with this subject but not your last time :). I have a soon to be 16-year old so I've swan these waters! These tips will apply to many situations you will face with a preteen and teenager:
1. Try not to jump ahead to the scariest scenario you can think of (in this case, she is going to be having sex). Or jump ahead to worrying about her whole life being ruined. Kids have the enviable ability to be "in the moment". Something that is attractive to them today (like this boy) will wear off in a week or so.
2. Please don't miss this opportunity to develop an open door policy with your daughter. It could very well be that she didn't tell you or lied to you because she feared how you would react or she feared disappointing you. If she can't tell you about an innocent "boyfriend" at 10, there are MUCH scarier things she won't tell you about when she is 15. Sometimes you have to bite your tongue so they don't think they will get a lecture (and by "lecture", this age group means any sharing you do of you opinion regardless of the amount of time it takes :) ).
THE QUESTION IS NOT IF WE CAN TRUST OUR KIDS, IT IS IF OUR KIDS CAN TRUST US. Ask yourself if your daughter can trust your reaction enough to tell you anything. If the answer is no, please go to your daughter and apologize. Tell her that you want her to be able to trust you with what is happening in her life - NOT because you want to tell her HOW to manage it but you want to celebrate or commiserate with her and, if she wants feedback, you'd like to share with her. Tell her you may have reacted negatively because you want what is best for her but that you know she is a smart, wonderful kid with a good heart and you know she wants to make good choices.
3. Allow your child to "try things on" within your watchful and cautious boundaries. Even though she has a "boyfriend", at this age they can't be together or go anywhere without a parent's permission and chaperoning. Sometimes this means that you have to hang out with them and listen to all the gusy gush - believe me, I've been there. Your "support" doesn't mean that you have to like everything or comment on everything. It just means that you are allowing your child to make some of her own choices while still under your protective wings.
We want our kids to learn lessons and feel consequences when the stakes are small instead of when they are teenagers/young adults and the stakes are much higher. I recommend BARBARA COLOROSA's teachings on this matter.
S., you are going into a emotion charged time with a preteen/teenager. Your daughter needs you as a ally -not a "friend", she still wants you to be in charge. It can be SO fun and build a life long bond between the two of you. Good luck and enjoy it!
T.