Oh mama, the uneven engorgement feeling is so real and honestly a bit worrying when you've had mastitis before, totally understand why you're anxious. From what I've gathered, pumping at 16 weeks is generally not recommended because stimulation can trigger contractions, and even gentle squeezing can do the same, so it's better to just leave it alone as much as possible rather than try to "clear" it yourself. The soreness and that "stuck" feeling on one side is likely just your ducts gearing up again, and the good news is that mastitis risk during pregnancy itself is quite low, the real watch point is after birth when supply kicks in properly. Do bring this up with your gynae or a lactation consultant sooner rather than later, especially with your mastitis history, so they can give you a proper plan before delivery.