Strange Shoulder Pain on Side Raise ??

(Symptoms at the bottom)

I'm 26 almost 27 and I've been playing tennis since high school, and I've been active in different sports activities almost every day, but after my early twenties began till my 25 I wasn't that active because I was more involved in work. An year ago, last summer to be exact I started playing tennis again, a few days a week and I was doing fine. Then at the end of the summer, I felt on my tennis serve that my shoulder hurts and it was getting worse every match. It wasn't that much of a sharp pain, but I felt as if my arm is getting weaker throughout the match. I even felt as the weakness/pain is extending to the elbow. But as soon as I stopped playing tennis throughout the winter it stopped.

Then this year in january I started lifting weights. I never did any heavy exercises in my life, so I started with smaller weights, and kept increasing the weights slowly. After a month or two, I was doing a few sets with about 80% weight and 4-6 repetitions. I was doing mostly shoulder and chest workouts. Then I started getting a sharp pain in my right shoulder. It kept increasing day by day with every workout. I went to a doctor and he prescribed me some pills diclojet for inflammation and an anti-inflammatory gel fastumgel. I went through the whole therapy, while I stopped working out, and the pain decreased significantly, but it didn't go away. I tried working out again and the pain was increasing again. Then I read about rotator-cuff exercises and I started doing them every day. It didn't help much again. I don't know what to do. I don't know whether it's the rotator cuff or not, because the symptoms are not quite the same as everyone describes.

So now my shoulder hurts ONLY in the following position: If I raise my arm on my side with my thumb pointing down. I can only make it about 45 degrees from my body, and then a sharp pain in my shoulder begins. Other than that I can almost do any activity normally even play tennis again but I'm afraid if I start playing regularly the pain may get worse.