thank you for your query..... it seems like unmindfully you have got a scratch on your back and that might have got infected to some extent leading to the vesicular appearance...it might pain a bit later but it is a minor thing... apply mupirocin or neosporin ointment over it gently twice daily for 4-5 days and it will heal... however if similar fluid filled vesicles appear in the adjacent area or in other areas of your body in a few days and it becomes painful, it might be some other disease like chicken pox or herpes infection.... in that case you need to see a physician for investigation and treatment.... hope that was useful... you can contact me for any further questions... regards