Could be just a pulled muscle, you may have coughed or sneezed too hard. Have you done an activity with a lot of repetitive movement that utilized thosed muscles? if you are a nurse you may have pulled it while movin a patient, if it doesn't subside after a couple days, maybe consider seeing a doc.
Does it hurt when you breath in? I'm thinking it could be inflammation in between the ribs like pleuracy.
Maybe an intercostal muscle pinched under the diaphragm? Try some bodywork; like Orthobionomy; maybe that can help. I understand the "nurse thing".