Um this is long lol. I usually try to keep things shorter than this... Anyway..
The weightlifting may be, like FA said, an overcompensation, which is common. I never got too much into the overcompensation things, but when I was trying to make being a guy work i did play up whatever few masculine traits I had. Like, I was good at sports, so that became my thing. it was the one way I felt I could interact with guys where I did not feel like I didn't belong.
And that was kind of my deal, I knew I felt like a girl, but I couldn't stand the idea of being a disappointment to my very conservative parents. Also it was the 80s/90s when I was young, and it would have been horrible if I told anybody how I felt, unlike today where it is better, somewhat. So I hid any anything about my nature that could have been even hinting at femininity with an obsession because I thought people were more observant than they actually are, and I just knew I people would know if I gave the smallest hint. Anybody observing me back then would have seen a guy with no visible feminine traits, but seemed uncomfortable in their skin, who was very distant and reserved and soft spoken, but was good at sports. Nobody probably saw any hints of being trans.
Meanwhile, I felt like yes, I am secretly a girl, but why does anybody have to know? I felt like in time those feelings would go away, or if not I could find a way to live with them. However, time went by and I found that instead of those feelings going away, I felt worse and worse about living a lie. It started feeling like my whole life was fake, and I didn't know if I was even real anymore, because if you live a lie long enough you start to believe it, and the incongruity between that and deep down knowing it was all fake began tearing me apart.
I became deeply depressed, I started hating myself and even being alive. I saw no way out, I had learned about transition but knew I could never make that work. I just wasn't strong enough. Without hope, it got very dark and very bad.
Somehow I found enough of a will to live to try transitioning, even though I knew I would fail at it. It is funny how living your life authentically changes everything. I started taking the steps and the depression faded (over months and months) and eventually was replaced with something I had never experienced in my life: sustainable happiness. It changed me. Transition has given me the chance to be the very best version of myself that I can be. I very much love my life now and love the person I have become through all of this.
You are an amazing mother to try to find out more and understand this condition, rather than try to control your child and "make" them match how you think of them, which is what my parents did when I finally told them. Your child may not be ready for presenting themself as female publicly yet, it's scary! I think the first step is to have them talk to a therapist about how they feel, preferably one that specializes in these issues. Then they just need to take baby steps, or whatever size steps they feel comfortable with, and head toward understanding themselves better. Probably they will end up in a different place than they imagine at this early stage, I know I did, so don't take anything they say about their sexuality or what surgeries or whatever steps they think they will take as gospel truth just yet. They have a lot to figure out still. The best thing is that they start figuring it out, and that they don't wait till it gets so bad that it becomes dangerous.