Instant breast engorgement, I think?

Is that even a thing? For example tonight I'm in the tub, I gently check my breasts to see if I need to pump before bed as baby has gone down for the night. Breasts are about half full...then I feel immediate tightening and the right one (my work horse) gets rock hard. I went from half full to instant engorgement, and now 10 minutes later it feels half full and is not as hard again (still in the tub, haven't pump). What is happening? I'm not leaking so no obvious letdown and this happens while breastfeeding too. Can't find anything on Google.