Grief can manifest in many ways and a lot of them are physical. I am so sorry you have this experience, because I know it is just terrible. Especially when it is your child, it feels all wrong on top of the grief of the loss.
whether or not this is fibromyalgia, though, is something you will just have to wait to see. If you had no such problems prior to your son's passing it's possible that those things will eventually stop. On the other hand, sometimes fibro is triggered by very traumatic events, and losing your own child, at any age, is certainly traumatic.
I suggest you do not make any assumptions or come to any conclusions about whether or not your physical problems are fibromyalgia, or anything else, until some time has passed. If you are still experiencing physical issues in several months or a year from now and you feel able to do so, then get the tests that are required for determining what is causing this. Without the tests you will not know, and it could be something else that it treatable, curable, or very serious. You will eventually need to know.
Right now, concentrate on yourself. Be very kind and very patient with yourself. Let yourself have all the feelings you have and all the time you need, no matter how long that is, to grieve. There is no right or wrong way to experience grief or to get through each day or week or month, as long as it is not something damaging to your own health. Find support for your grief. I know joining a grief support group, both in person and online, helped me a lot, as well as talking with anyone who was able to listen to me without judgement or trying to "fix" me. I hope you are able to find that for yourself.
And you are welcome here even if you don't know whether or not you have FM. Judgement is not allowed here, and people here are kind. We will help and support you however we can.