Many different things can cause left arm pain, so you should see a doctor to determine the cause. Most of us freak out and fear we are having a heart attack when the L arm aches. While this is a legitimate concern and should be considered it is not usually the cause.
Acid reflux (GERD) often causes aching in the L neck and arm (I don't know why). If you have acid reflux, or think you might, consider this as a cause. Also a number of muscular or nerve problems such as pinched nerves or muscle spasms can cause aching in the arm.
note: I'm not a doctor, but I have experienced this myself many times and have seen a doctor and done extensive research on the topic. Don't take my answer as medical advice, but as advice from my own experience and research.
Sounds like your heart to me, but you may just be misaligned in the spine. I'd go see a doctor.
waht can cause my left arm to cramp and numbness in my left fingertips
A heart attack.
Those are all GREAT indications of a Heart Attack so I would say its time to head to the Emergency room, Cardiologist or call an ambulance. .
Left arm could very easily be a heart attack, that was what I felt along with hot flash and a body tremor. Right arm could be a pinched nerve or strain. If it is your left arm, get to a hospital ASAP. Don't think you can't have a heart attack that you are too young or too healthy or that it just can't happen to you. GO!
the muscles starts streching every moment you sit down
An ache in the arm that does not go away should be evaluated by a doctor. There are many reasons an arm could ache so it is important to be able to tell the doctor what you have been doing prior to it hurting.
It could be just a muscle strain if you have recently been lifting something heavy. But, if it has been hurting this long I would probably go see a doctor. Pain is never a good thing in most cases.
It could be a sign of a broken rib.
Possible broken a-arm, or bent wheel.
Your arm aches because you might have been sleeping on one arm and putting to much pressure on it. Later, it might feel numb, or might ache. Also, it can happen to both of your arms.
That would be tattoos on the arm opposite to the right....