Fitness

I don't pistol squat often but I feel this too. It eases off for me in subsequent sets

Ya I think that's reasonable, you are working on both legs when you do a pistol squat. Might be good to stretch more though, if you are just getting into lifting/body weight exercises, I would stretch daily/before/after working out. Stretching is one of those things that doesn't get enough credit until you do it and see the effects, especially as you get older.