So sorry to hear you are going through all this worry.
I know it's easier said then done but please try and not worry too much and occupy yourself with nice things like lunches with friends, time with family anything really to take your mind of it.
I know I wish I done that, you see I went through the exact same at the back end of 2007, I too had a mass the size of a tennis ball that seemed to grow just before my period, it was also the only time I had any discomfort from it.
I had all the ct, MRI and X-rays along with blood tests and then surgery on 26th January 2008, I then had a very long nail biting wait for the pathology results, all came back fine, that was my very first experience with endo and as they say the rest is history!!
I have since developed further masses and had further surgeries, it is not very common to have that type of endo, but I know of a couple of ladies on this site with it.
I Really hope all goes well for you