Maddy, It could be a number of things including gas. If you have a fever or the pain increases from mild then you should visit your family doctor.
Put gall bladder into your search engine and check out food you can eat and food that triggers the pain its all to do with the eating.
Kind of sounds like your gallbladder to me
It's really hard to know unless you have an MD respond or someone with similar symptoms.
You should take your temp and do you know how to count your heart rate or pulse. A rapid heart rate and fever may indicate that you have an infection somewhere in your body.
You should also consider whether or not your bowel habits have been regular as well as making urine. Is your urine cloudy or pink? This could indicate a kidney infection or stone. I bring up kidney because you say you've felt the pain in your back.