What Causes Indentation On Arm Along With Severe Pain?

I was wondering if anyone could shed some light or offer any advice or if anyone has been in a similar situation about 6 months ago I started to get a very painful dull ache in the top of my arm simple things were making it ache a lot so when I looked closer at my arm I saw that it had dented in like the muscle was missing or sunken it was pitted so I was quite alarmed by this and headed off to the drs who could not explain it ..they sent me for an xray which was clear and told me to return for blood work it was clear, it was still aching ...the ache subsided and so I did not carry on with any thing else . since then I have had some twitches which lead me to be very anxious about ALS ...im worried I have some muscle wasting thing or cancerous growth drawing on the arm ...