Any time you have unprotected sex you are at risk of getting pregnant even if he did not ejaculate inside you. Though the chances of getting pregnant off pre-*** is VERY unlikely. You bled, so I would take that as a good sign that you are not pregnant.
Maybe the stress of thinking you may be pregnant made your period light, or maybe because you missed those few pills, i don't know I've never been on them. If you are very worried, see your doctor, but the changes of it are VERY slim so I don't think you have anything to worry about