Today I ate too much peanut butter will I be fat? And is it bad for health

Not for one day..but. In the US of A, we often eat too much of a lot of junk foods..so moderation is key. If you are not heavy and generally eat well, exercise regularly etc..then a bit of peanut butter excess once in a blue-moon is not going to make you fat.. So, use your common sense.. Stay well and healthy... Good luck.