((JustWannaDisappear))
I did nurse both of my girls. I got mastitis once (about one month into nursing my first daughter) ~ which I recall as being a dramatic difference from "normal". The breast became hard and HOT, fever throughout the body. All in a matter of hours. Pretty intense! The answer to that problem is antibiotics and getting the excess milk out to relieve pressure.
Years after I had stopped nursing my daughters, (about 4 years) I found a lump in my breast. It lasted for months, despite my menstrual cycle. I saw my doctor, and mentioned it to her. She felt it, and sent me out for x-rays and a mammogram. The x-rays showed a relatively small mass. The mammogram, however, squeezes the breast so tightly, the mass was squished apart. My doctor figured that the mass in my breast was dried milk, and the pressure of the mammogram squished it apart.
I have felt my milk ducts kick into gear a few times (for seconds) in the last few years. Not incredibly unusual, apparently, but it was a surprise to me. My doctor was surprised too though.
That's my experience. I'd recommend calling your doctor for an appointment. Doesn't have to be an OB/GYN. The part that your pain is pretty intense is what would make me act now, rather than wait a bit. It's better to get answers, rather then wonder & worry, and let the pain levels intensify.
Best wishes to you!