Spam issuing from my comcast email account to people in my address book?

This is the second time in six months I've had this problem. I use strong passwords; i don't click on suspicious links; i don't use my email password for anything else. I have a macbook pro with OSX 10.8.5 and run all updates as they come out. (also have an iphone 4S and use email from there, too; not sure which might be the source of the problem). I use the generic apple email on the macbook pro/iphone, sending email through a comcast account. I could still access my email after it was compromised and i immediately changed the password when I became aware spam was going out. I have checked my email preferences to make sure nothing's being forwarded; it wasn't. I would appreciate guidance in what else i should do, because i'm annoying the heck out of my contacts and really want to make sure this is resolved permanently.

(*please use small words: i'm entirely tech illiterate, as I expect is probably obvious from what I already wrote. Telling me that i just have to snarfnigle my 3-bit gizziwigit is not going to help me at all. "go to your *** menu and select ***" is something i can do.)

Added (1). thank you for the help. I learned about it because it went to at least some of my contacts, not sure if it went to all of them as it seems to be dribbling into people's inboxes slowly over a couple days. Hopefully the changed password has taken care of it but i'm just anxious to do whatever else i can.

I'm not sure it's coming from your account. Your email address could be spoofed, there's not much you can do about that. You state you became aware spam was going out. How did you become aware.Were your email getting sent back to you? Or did people on you contact list tell you about it? If everyone of your contacts is getting the spam, then it's probably coming from you and if you changed the password on the account and it's still happening, then your computer is infected.Since you are on a Mac that's not likely in less you have been downloading from untrusted sites.Odds are someone on your contact list is infected and it's spoofing your email so it's harder to find the source. If one of your contacts in infected the only thing you can do is let the people know who are getting the spam and tell them they need to check their computer for malware.