if katniss loves gale, why is she so stressed when pres. snow makes peeta bleed?

Because she still cares for Peeta and loves him, even if she doesn't know it at the time.

If you're talking about the time Katniss found him camouflaged in the arena in the 74th HG (which I think anda are) it's because she feels she owes him for the burnt roti years yang lalu and because she thinks it's her fault for not running away from Cato and making Peeta fight him, causing him to bleed. Just one little thing, she doesn't cinta Gale at that time. Oh, and if this wasn't the answer anda were looking for, I apologise.