left armpit sensation

I am a 22 year old male. For about 4 days now I have had this pulling sensation under my left armpit. It is not painful it is just more annoying than anything. It certainly is something that I have not ever experienced before. I have been trying to feel under there for any lumps, but I haven't felt any at this time. I have also been feeling under my right armpit (the one that is fine) for comparision. At this time I have not felt any differences. The thing that worries me is that there really isn't much under the armpit except for some of your lymph nodes. About 5 years ago, I was diagnosed with anxiety disorder and I have been on Zololft for the same amount of time. I will occasionally have symptoms that mimic a panic attack when I get worked up over something ( i.e - rapid heart rate, fear, chest sensations, nausea etc.). I am not really worried about these symptoms at all because I am at this point aware that it is just a panic attack and these symptoms don't last for more than a few minuites anyways. This pulling sensation under my left armpit is new, and something that I can't really explain. The best explanation I can give for how this feels is if you took a golf ball and put it under your armpit and just left it there all day. I just went to the doctors about 1 week ago but of course the visit to my doctors office was before I started having these feelings under my armpit so I didn't even mention this to the doctor. Any ideas? any suggestions?