Elin,
I am too going through the exact same thing you are mentioning. I have a pretty much constant pain in my left thigh for the last couple months. It's not severe but it is almost always present. It's like a tight almost crampy feeling that feels like it needs to be stretched out, but can't be. My left quad and calf are also markedly smaller than my right (but I am right handed), and my left leg twitches more than my right as well.
I have had BFS for the last 5 years and am still going strong. I chalk this episode up to just another new symptom, which will eventually be replaced by something completely different in a month or so. My BFS is good like that, just when you get bored of a symptom, it changes things for you.
In the interm I have found that ibuprofin does help alot, so I guess it can't be anything too serious. LOL. Good luck, and keep your chin up.