things to consider:
the more people that saw the flop, the more likely someone has a stronger hand. raises show alot of strength in multiway pots but not so much when headsup.

the exact pair you have makes a difference. TPTK with AK on Axx board is different from A9 on a 987 board.

what people think of you...do you have a good or bad image? if you have been playing tight and someone raises you (and you think they have some idea that you are tight) then use caution.