My daughter is 3.5years and I simply tell her that I am feeling sad, or cranky etc. I also usually say "you know when you fel a bit too tired and you feel a bit cranky, and sometimes you need to calm down and relax... well mamma is feeling that way" I always make it
very clear that it isn't her fault and I asked her to help me feel better by doing xyz.. which might be have a snack while I put a movie on for her and have some quiet time (we don't watch tv much) or to play outside, or get some books for us to read together.
I hope that you are feeling ok