That does sound llike an allergic reaction.

Forum member Surasak reported a similar issue that plagued him on a number of logged occasions relative to a later melanotan II regimen of his (he hadn't experienced any issues previously). See: http://melanotan.org/cgi-bin/yabb/YaBB.pl?num=1233084402
He unfortunately continued to take the substance by using antihistamines despite having previously developed rashes. He subsequently developed some serious health issues. It is not clear if his regimen was the cause of his health problems but given that he had experienced rashes (prior to using antihistamines to overcome them) with a particular batch of peptide, his explanation is plausible.
 
My recommendation would be that either you don't use melanotan II ever again or if you do decide to then don't use any of the substance that you used which appeared to cause the allergic type reaction you experienced.  
Pure non biodegraded melanotan II combined with non-expired bacteriostatic water injected with good quality non-expired syringes should not cause a person to break out with a rash.  
 
Also, if you have not already done so it would be advisable to seek the counsel of a qualified medical practitioner about what you experienced. If you do go, try to be as upfront as possible about what you believe caused the issue.  
 
-Scott