Wonkylegs, looking at the potential side effects of Pregabalin, "weight changes" are note, amongst lots of other things, including bloating, constipation and a plethora of gastro challenges.
Could your weight be bloating at all, or have any of your other functions changed? Is your clothing tight? If so, where?
I have a lived with constipation long term, made worse by hypothyroidism, so I can see my weight vary quite a bit, just because of what I have "on-board". Ahem.
Don't be too hard on yourself. In the scheme of things, 8lbs isn't a huge gain, but I do totally appreciate you don't want that trend to continue or be established.
Edited to add the url for the BNF listing:
https://bnf.nice.org.uk/drugs/pregabalin/#side-effects