Why do I feel like such a mean Momma?

The last couple of days everything my kids do just seems to irritate me. Both yesterday and today my 8 year old has spilled every drink she has been given. I don't mean just a dribble...I mean completely knocked over, on to the table or all over her food and in the plate. Do we NEED to go back to sippy cups since you can't seem to hold onto anything?..I asked her in a loud snappy tone. When I came home from work today there were like 14 cups in the sink. I was only gone 4 and a half hours...why in that amount of time, do we need to use 14 cups? (No I don't leave my kids by theirself at home, they have their Dad with them.) I just feel so grumpy and mean lately. Maybe its PMS...don't know LOL.