Hi, Marianne, and welcome! I know that back pain. Gentle stretching exercises do help but that spot is difficult to reach with the exercises. There is one thing that does help stretch those muscles that are most likely in a spasm. Try standing up, bend at the waist so you are facing the floor, let your arms hang down loose, and then start swinging your arms in all directions...even in circular motions. This works those muscles up around your shoulder blades. Also do other stretching exercises.
With the flu, you probably are coughing a lot and that easily could have caused this excess pain. I use a product here in the States called Muscinex DM. It loosens phlegm but you only cough once in a while...but it is a productive cough. Other cough medications can make you cough your brains out! LOL It's the DM (dextromethorphan) that causes the "selective" coughing. I'm not a doctor but am sharing what helps me.
Moist heat helps a lot. We have something called a Bed Buddy that we can get in drug stores and other places but you can make your own by taking a tube sock, filling it 2/3's full of raw long grain rice, and tying a knot at the end. Pop it in the microwave and it will give off moist heat due to the moisture in the rice! Oh, does that feel good!
If your flu is getting better, try to keep moving, too. If you sit or lay too long you will be stiffer and have more pain, too.
As Debbie said, be sure to read Fibro 101. It really has a lot of great information on fibro. I've had fibro for 25 years, also, and am still learning new things daily!
I'm looking forward to getting to know you. Don't hesitate to ask questions because we are here to help you. I do hope you start to feel much better and I hope to hear more from you soon.
Sherrine