I've never had that exact experience. Of course, my boys are a bit younger.
I did have the experience of having real wonderful, intense sex with my husband early one morning (complete with him begging me to let him cum), then settling down for some cuddling only to feel a very small hand slapping at my thigh. My then-5-year-old son had heard some noise and come to see what we were doing. I have no idea how much he heard and saw, but that's when we started locking the door at night.
A lot depends on the maturity of your daughter and your relationship with her. Most 14-year-olds have at least some idea of sex and at least comprehend the idea of masturbation if they aren't already experts in the area.
It may be a bit embarrassing to you (and her), but talking to her is not a bad idea. Many people still get the idea that masturbation is dirty or bad. Assuming that you are a positive role-model for your child, this could be a way to improve her image of her own body and her attitude towards sex. Think about the things that you hear about women who have vibrators and/or dildoes - most of them are not very positive. The words "slut" and "whore" come to mind very easily. This is definitely not the image you want her to have of you or of herself.
I'd guess that she won't really want to talk about it. I'd also guess that she's probably mentioned it to one of her girlfriends already (remember how much you shared with your best girlfriend). This is probably not where you want her to get her advice on sex when the topic moves to birth control and who/when/how to start having sex.
Hope this helps. I'll say a little prayer for you.
Ahhh the little joys of mother hood nobody ever tells you about!
I once had the good fortune to have left my vibrator on the bed after use and then forgetting totally that it was there sending my oldest daughter in to retrieve a dirty mug for me. Imagine my suprise when she came out with the thing in her hands and asked me with the candor that only a (then) 5 yr. old could possess if I didn't think this thingy looked "an awful lot like a boys pee-pee".
Ugh that was a day! I didn't want to go into to much detail as she was only 5 at the time, but I knew I had to explain it to her somehow. I ended up telling her that it did in fact look an awful lot like a mans private parts and that it belonged to mommy and was for private use. Thankfully that was enough to quell her curiousity as her knowledge of sex and masturbation was none. In your case however, you have a child who knows what was going on. I would try being honest when you talk to her. By avoiding the topic or making it seem taboo you'll only encourage her to go to her girlfriends or other unreliable sources for info. Usually a short, honest and frank response will do the trick. And who knows, you may be doing her a service. As another poster mentioned at 14 she may be masturbating herself and knowing that it's a normal thing may help her feel more comfortable about herself in the long run.
I have no children, but it wasn't that long ago I was 14. How much does your daughter know about sex?
I really think that if she knows enough to know what you were doing (and at 14, she probably does), there is no need to talk to her about it. It's only going to embarrass both of you.
I think you have to approach this based on the relationship you have with your daughter and her knowledge of sex. do you have open and frank discussions with her, have you ever discussed masturbation with her, do you know if she masturbates. If the answers are yes therer may be no need to discuss anything as she already has knowledge. If not maybe it is time to have thses discussions. Be open with he, let her know masturbation is a very natural activity. Do not give her the impression you were doing anything worng or doing something you are now embarrased about, be positive, and don't make it into a big deal.