I start crying all the time for some reason, why?

I seem to always start crying. Everyday there is something little, that makes me cry. I don't know why because it's not even like I'm that sad about it, I just cry. For example just now I heard my mum and sister talking on the landing outside my room so I went to see what was going on (I am nosy) and my sister looked at me and shouted "Go away!" I ran back into my room crying and I can't stop, I don't know why. Also sometimes if I get hungry and I go downstairs to the kitchen and theres nothing to eat, I start to cry. It's not like I'm even that upset about it. I do it if I'm with my dog and he jumps up and I don't tell him to get down, then my dad tells me I should, I go up to my room and start crying as if I'm ashamed of myself, like I've done something wrong. I don't know why I do this because I'm not a wimp. I don't cry if one of my friends says something mean. I would really like to know why I'm always crying over things when they don't actually upset me that much, Thank you in advance.