I have a large lump on my left breast scheduled for a mam on friday, tonight I had sharp stinging pains radiating across my breast and nipple?

See a breast doctor. Majority of the breast lumps are benign like beast cysts. An ultrasound in addition to mammogra can help determine if a lump is a solid or a cystic lump. Cysts are almost always benign and sometimes they are painful. Solid lumps can be benign or cancerous. After you had your mammogram you definitely need to see a doctor, preferably a breast specialst even if your mammogram report was negative.
You may or may not. Some breast anormalities on mammogram can remain and be seen on future mammograms. If they remain stable there is less concern over them. Some abnormal findings may subside. Every mammogram should be compared to previous ones to assess any new changes or its stability. Please also remember that about 10% of breast cancers may not be detected by mammogram. A clinical exam is also needed since some breast abnormalities can be detected by your physician and not the mammogram.
Ask for pain pill. I assume your doctor is suspecting something, since you will have mammogram in 2 days, just take some pain medications for few days until they know whats causing the pain.If the lump is large , you better schedule a biopsy after discussing with your doctor.In your case i would prefer a sonogram .