Teens today are sexually aware and active at younger ages than our parents ever dreamed possible. Clothes, ads, internet and other girls make sexuality a main steam topic, and too often, activity. Only you can assess whether your daughter's behavior is indicative of the usual culture or indicating that she is engaging in some risky behaviors.  The "hickeys" make me lean toward thinking she is taking some risks.

 Your daughter's behavior, as all behavior, has underlying intention, whether the person is aware of it or not.
What could this behavior mean?

Is she seeking attention?
Is she telling you she needs your attention?
Is she feeling bad about herself?
Is there some larger problem she is trying to solve with "acting out?"

In trying to assess the level of problem that you have here, do consider other behaviors.  How did she do in school? Was there a drop in her academic performance?  Were there other signs of problems in her attitude, her relationships within the family?  All these areas can be taken into consideration when deciding what is going on and what to do.

DedicatedCo suggests talking to your daughter and listening to her.
This is certainly good advice.  However, it may not be as easy as just sitting down and explaining to her "how-it-is."  It sounds like strong peer influences are at work here. It is good that her best friend's mother made you aware of the problem.

Here is what I suggest

In order to "listen to her," spend time with your daughter.  You may already be doing that, but if not, do.  If you are, try to spend a little more time.  Give her an opportunity to tell you what is going on.  Listen carefully beneath the story line to see if you can get info on any of the above questions.

Set limits. At fourteen, she is young enough to have rules.
Set rules that you feel are appropriate and correct for your family regarding how she dresses and how she behaves. Have limits for how late she can be out, and who she can go out with.  While you cannot control everything she does, you can set limits and have reasonable consequences if she breaks the limits.  Be consistent.

If you think your daughter may be "acting out" some adolescent problems with her behavior, you might want to consider taking her to a therapist who deals with adolescents.

Whatever you decide to do, it is good that you are taking notice.  Do the best you can to talk to her, listen to her, and set limits for her.  However, be realistic.  You will not be able to control everything she does.  A solution may take time. Don't expect miracles. You may need to try a couple of these suggestions, and use them over time.

Another thing you might try is developing a support group of mothers in your daughter's age group. Most mothers of 14yr olds worry as the hormones start to heat up.
There can be strength in numbers. There is certainly lots of support to be gathered from others who also are dealing with teenage girls and their issues.

Lots of luck.
Bette J. Freedson, LICSW, LCSW, CGP
Clinical Social Worker
JustAsk Expert
http://www.singlemomsos.com/index.html