Is this serious? What should I do?!

This sounds like you may have what is known as Impingement Syndrome. Some people know it as Swimmers Shoulder. You may have noticed that it hurts worse when you lay on that shoulder or try to raise your arm above your head. You need to see your doctor for this. He can give you some exercises that will make it feel better. He may also do an MRI to see the severity of the impingement. I have had this in both of my shoulders and had cortisone injections in them which did make them feel much better. At times my shoulder would hurt so bad I was almost sick on my stomach! This tells you more about it.
http://en.wikipedia.org/wiki/Impingement…
It could also be bursitis. The bursa in the shoulder joint can get inflammed and it is also very painful and has many of the same symptoms of the impingement syndrome. Call your doctor and have him check it out. This is the only way you can get a definitive diagnoses.

Nurse for 30 years.

Get a doctor to take a look, do you really think some random loser on the net is going to be able to diagnose you?

I'd get it checked out by a dr. I have never heard of anything like that