My MRI results: Mildly heterogeneous myometrial echotexture without a well circumscribed mass. Could this explain my abdominal and lower back pain?

Probably not. It may be within normal range, difficult to assess without the images and the description of "mildly" heterogeneous echotexture. Also, that is an ultrasound description of findings, not MRI. Which did you have?
Fibroids/adenomyosis. The two most common causes of a heterogenous uterus are uterine fibroids, which are benign muscular growths in the uterine wall, and adenomyosis, which is a proliferation of the normal uterine glands into the muscular wall of the uterus. Both of these conditions are benign (in the cancer sense) but can cause pain and gynaecologic symptoms.
Pelvic MRI. Here are some general comments on purpose of a pelvic MRI: https://medlineplus.gov/ency/article/007355.htm For a specific interpretation and whether an answer has been found re your pain please consult the doctor who ordered this test for you or a gynaecologist licensed in your area as there are a variety of possibilities here.

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Mass on the back of my neck, attached to R side of my spine. Had stat MRI. Have lower back pain-4 months, hip/leg pain. Does this sound like a tumor?

MRI. What did the MRI show? The neck lump could be unrelated to the pain in the lower back, hip, leg. May be a lipoma which is a benign fatty tumor. Read more...