Oh no! Is it painful, hot to touch? Does the red mark run from outside of breast down towards the nipple? As in, the same direction milk would flow?

You can get mastitis without an infection apparently, but it is quite uncommon. Most mums get mastitis with an infection which leads to flu like symptoms, hot and cold flushes, fever, nausea. If you start to feel sick, you need ABs.

Keep up trying to clear the blockage. Express if you need to, to ease the discomfort. Hang in there.