Hmmm. Just to ask, "Am I a bad mommy?" reveals a bit of guilt, I think. It's not likely anyone will respond in the affirmative to that question, and it's something almost all mommies want to hear, so, NO! - YOU ARE NOT A BAD MOMMY! Even going back to work full time, three jobs, 8 days a week wouldn't make you a bad mommy.
Ok, so now that that's done ;D let me offer an alternative question for you to ask yourself, as only you are capable of answering: Am I the best mommy I can be?
For some, it seems their answers reveal they can only be the best mommies they can be if they are working outside the home. For others, it seems their answers reveal they can only be the best mommies they can be if they do not work outside the home. While I usually abhor post-modern flavored, what's-true-for-me-isn't-necessarily-true-for-you-type relativistic cr*p, it is almost what I feel to be the best case here. The caveat being, of course, is to ensure you honestly embrace the virtues God has entrusted you to hold when it comes to raising your children - things like grace, generosity, patience, kindness, and love - when you are making your decision. There are also important lessons you can teach your children through life decisions such as these like the good of putting others first, the good of taking care of yourself, the value in delayed gratification, the value in taking risks and living without fear, and the importance of doing things God's way as best you can gather from prayer, reading the Bible, and walking with Him. You can do all those things, teach all those lessons, and embrace all those values whether you work PT or stay home. And, like I said, only you and your family can know which path is most likely to give your heart a "YES! I am being the best mommy I can be!" answer at the end of the day.
HTH!