Why have these girls given me the cold shoulder?

I would say it is because that "friendly flirting" you were talking about probably scared her into assuming that meant you liked her more then you had intended. If your friendship with a woman isn't flirty (strictly friendship), sassy, sarcastic etc then to start being like that will throw us off and we will assume things.

Probably something like April said but also maybe they're playing some kind of game with you ? Or maybe they saw something nasty that you did? I mean - girls get disgusted by almost everything that a guy would do - that a girl wouldn't want him to do. Did that make sense? lol but at least that's how I am. Or maybe they heard something about you?

The only reason I would consider doing that to a guy is if he was too overbearing- acting like we're BFF and always in my face. Although I'd never ignore someone, I'd just call them out :)

I don't know how close together these two incidents occurred, but I'd give the awkwardness time to die down and probably drop them an email and say "You know... I felt really crappy when we just stopped hanging out. I sincerely hope everything is going ok with you. Take it easy." Put the ball in their court, then let it go.

Ok, you answered you own question buddy. Your in what I like to call the "Best friends Zone". Its a natural fact that no guy/girl can be friends with another without at least one of them having sexual feelings for them- no matter how minimal.

In this case, you are the one that has the sexual attraction to her. The problem is, you acted like you were friends for so long, that it has eliminated (in their minds) any chance that you are boyfriend (or even sexual) material.

They have placed their trust in you, by opening up, and becoming friends with you. All of a sudden, when you start flirting with them, they feel like you have been lying to them/using them just to get into their pants.

This happens all too many times, guys wait too long to start making their move, and get into the "best friends zone". Once they get comfortable with the girl, they start moving in.
This pisses the girl off, because now, (in their opinion) your ruining the best friend relationship.

How can you fix this? YOU CANT! Move on to the next girl, and make sure you make your move earlier to avoid Best friend Zone!

If you DO end up doing this again, remember, a guy can NEVER have too many female friends. The more good looking female friends you have, the more you are "accepted" you are in the hot girl community. If a hot girl sees you hanging out with other hot girls, your social value has gone up. Its easier for hot girl to accept/want a guy if he's surrounded by hot girls.

There is nothing wrong with being in the best friend zone, but you have to learn to recognize when your there, and not cross back over the sexuality line.