Someone using my Imessage to send spam to other phones.

Recently, my imessage was used to send a message to 15 other people. The last phone number was a gmail account with a long and random sounding email at gmail. The message was a long random string of text, letters, capital and lowercase, with numbers. It ended with .surf.

i do not know who is sending these from a bogus gmail. Who do I report this to as spam or malware sent from my iphone . I did not send this. Should I tell apple or gmail? I do not want someone to call my number shouting profanity at me or coming to beat me up. Should I get a new SIM card? Already changed all passwords in both accounts. Thanks