More than likely, it's just because everything is so much funnier when accompanied by good friends. My best friend and I will laugh at the stupidest stuff when we're together, especially if we're in a weird mood, so I don't think you should take the giggling thing too seriously. Just brush it off -- they're probably just having fun. Maybe the elevator girls giggled at you when the door opened because they were surprised to see someone there or something.
I'm sure in some instances, the girls are just being bitchy. In those cases, you should brush their careless actions off, too.
You can be as sexy and confident as you KNOW you are. Think sexy and powerful and you WILL be sexy and powerful. Take life by the balls and ignore anyone who doesn't like it or you. Life is too short to worry about what others' think.