I wouldn't worry too much about it, if that's any consolation. I mean, you're obviously in distress, and it's not my intention to invalidate that. But it's natural to have not-so-nice thoughts in difficult situations. You're human, after all. We can only take so much, and motherhood is stressful, especially with infants, especially with teething infants. I wouldn't worry about it too much, again. It doesn't make you a bad person. It just makes you a person.
If you do, however, find yourself crossing over into resentment and find it continues on, and is interfering in your bond with your child and you start to question your ability to care and nurture him, you may want to consider getting help (a counselor, someone to talk to).
I wish you luck.