absolute classic plugged duct, you need to clear it, the white bit, chances
are you are going to need to attack it with a needle (sterile!) once you get
rid of that bit, a little bit of pressure and the milk will be shooting out.

sounds like you probably don't have mastitis yet, but it can turn into it
fairly fast, some people would suggest other methods before grabbing the
needle, but I personally think you're more likely to get mastitis by waiting
than by getting it sorted now with a sterile needle

Anne