Yeah most likely someone got a hold of your address book and is just spamming your friends, unfortunately dont neet to hack your gmail todo that (there are a tons of ways of getting your address book).

I would say to check the header of your spam e-mail on your friends mail box and see where the e-mail came from, if it was from google you might have some issue (maybe you gave some app or service access to your gmail, or have pop/imap enabled), if its not from gmail then dont stress about it, sooner or later the spam filters will pick up that its not from you ^_^ .oO( yeah dont hold your breath on google helping you on this, they really dont have any support! )