My muscles feel weak

[answerme123]

Im 15 years old, and lately i've been feeling kind of weird. My leg muscles and balande feel weak and bad. I can of course walk and move my legs, but they feel weak and I can't even ride my bike for 5 minutes before my legs start to hurt. I also get "cramps" in my lower back sometimes (not very often tho). When I get these I can't really move for about 5-10 seconds and my back stings and kind of stretches in this weird position. I went to the doctor for this and he said that I get this because I'm growing but I don't know anyone else who gets this. I'm also feeling a bit "foggy" in my head. My memory is kind of bad and I have a hard time just like remembering things and "being alert". Does anyone else have this problem and what can be the cause?

[answerme123]

Wanted to add that I've also started to have small speech issues like stuttering and kind of mumbling the words. And sometimes I get this kind of pain that kind of feels like a nerve is in pain or something. Only in my legs.