Why do my muscles feel weaker?

Okay. I’m going to start by saying that I know this is a long shot of anyone having a medical background/insight into what my issue could be. So, I’ll try to provide as much info as I can. I’ve had lower back problems lately. It hurts it I stand for longer than a minute or so. So things like brushing my teeth, washing my face, etc. become harder to do. The pain isn’t as bad as it was at first, since I am now on muscle relaxers, but it has not gotten better overall. I also have discovered that if I put weight on a certain leg while standing, it will start to feel numb. (Also, I haven’t injured my back in any way that I am aware of, but have had recurring back problems for years. I tore a muscle in my lower back when I was 14, and I had to go to physical therapy.) I’ve been to the doctor, and she said I have hyperreflexia and clonus. (I don’t exactly know what clonus is.) She wanted to make sure that the hyperreflexia and clonus weren’t related to my back problems, so I had an X-RAY, but it came back clear. My doctor re-examined me a couple of weeks later, and my reflexes and clonus had not improved, so now I am being ordered an MRI to make sure I don’t have a slipped disc or something like that causing these issues. I have my MRI scheduled, and lately, I’ve noticed that it feels as if my leg muscles are weaker. I’ve always had trouble walking, as I was born prematurely and didn’t walk until 18 months, but now it feels harder to walk in general.
In summary, I know it’s unlikely anyone will have any answers or insight, but I’d appreciate it if anyone has any ideas as to what could be causing this.