Never been to that specific mill but the pattern you're describing - that heavy pressure behind the eyes, the sense that something is watching from a fixed point rather than moving around - that lines up almost exactly with what I experienced at a derelict textile mill outside Rochdale about six years back.
What gets me about these old industrial sites is they arent just "haunted" in the traditional sense. The sheer amount of human suffering concentrated in one place over decades, the machinery, the accidents, the child labour in some cases - its like the building itself has absorbed something and it hasnt finished processing it.
Did anyone with you feel it at the same time, or were you alone? That detail matters a lot to me because simultaneous perception across multiple people is one of the harder things to explain away. Would love to hear more about the specific layout of where you were standing when it kicked in.