A rash is a potential side effect of Clonidine however, it usually occurs when you first start taking it (an allergic reaction). It does not mean it can't happen later, but just not very common. If it continues,and/or gets worse then you will need to tell your parents. My guess is that it might be from your anxiety (if you still suffer from it). When these type of things happen you have to be a bit of a detective. For example, what was different about that day?, did I use a different soap (including laundry) or perfume?, did I eat anything different that day?, was I under additional stress that day?,different vitamin or supplement?,etc. Make note of it so if it re-occurs you can look for something that makes both lists to try and find the culprit.
I hope this helps, feel free to ask if you have additional questions.
Are you on anything besides the clonidine? It is important for me to know all your meds before I can make a assessment.