Everyone's on the right track. You can get the equivalent of "tennis elbow" from any kind of repetitive motion of the elbow. But keep that doctor's appointment. He/she will probably have a treatment for you that will speed up your healing and may not require you to stop sewing.
I keep getting "quilter's elbow" and found that if I avoided quilting my big quilts, like the king size, on my small machine, I don't flare it up. And after that last queen sized one, the queen size is going in the same category. I will send these quilts out to be long arm quilted. That's because I've learned to do piecing without causing problems, and all other things quilting, except for all the pulling and holding on these larger quilts when I quilt them. So I send them out, quilt the smaller ones myself, and my problem is solved. Hope you get relief for yours as well.