I have a hard swelling in the left side of my neck that the doctor said is a swollen gland and didn't seem too concerned.

I think its around the sternocleidmastoid.

A few days before I noticed it I had a filling at the dentist and two injections in my left cheek. After this I had quite a lot of pain in the left cheek area and the doctor thinks the gland swelled in response to this. It has now been around 3 weeks and it is still swollen. It is rock hard and doesn't move, and feels like it has muscle on top of it so I cant really get a good grip on it. I don't think it has changed size, it just feels exactly the same. I just can't stop worrying about it and wanted to know if anyone else has had a similar experience. My main worry is that it feels rock solid. Its around the size of my fingertip, probably about 1cm or so.