Could I not be able to have children?

No I don't think you need to worry! Brown discharge, being a few days late, or even taking the morning after pill don't mean you wont ovulate again. Honestly don't worry about being late, even people who usually ovulate very regularly will occasionally be late. And bearing in mind that you hit your body with a tablet worth of hormones the other day I wouldn't be surprised if it affected your cycle a little. The brown discharge is a little more abnormal. I would advise always getting an STI test after every unprotected sexual experience. However don't panic as sometimes a bit of brown discharge can be as a result of periods. Fresh blood is red and slightly less fresh blood is a kind of browny colour! So my advise would be not to panic and go for an STI test just in case :)

All the best :)