Possible Pregnancy

Go to the doctor :\ Hope it wasn't a one night stand...could be an STD? Sorry, I think you should just go to the doc to be on the safe side.

If you need an answer, go get a pregnancy test kit and use it.

Here's info on the morning after pill and how it should be used:

http://www.mayoclinic.com/health/morning-after-pill/MY01190

Dizziness and nausea are considered "presumptive" signs of pregnancy, which basically means they don't tell you anything for sure. The next best level of evidence, considered "probable" signs, are usually not known until later on; that is,except for home pregnancy tests, which can be used anytime. The best level of evidence for pregnancy,considered "positive" signs and a definate pregnancy, require a visit to your doctor.

Now, as far as the headache and discharge go, they might be related to hormonal changes (however the discharge is unusual). They could also be some other things. I would recommend going to see a doctor and putting your mind at ease.

PS: I know I look like a dufus in my avatar, but I do know what I am talking about. ; )

You don't mention where you were in your menstrual cycle on the 18th when you had sex. If you were mid-cycle and presumably fertile then waiting 5 days to take the morning after pill was probably too late. They should be taken 12 hours apart and the first one taken within 72 hours of having sex. The headaches you're experiencing can be a side effect of the Next Choice pills. The earlier symptoms could have been from any number of things, including being stressed about being pregnant. The discharge could simply be premenstrual discharge if you're nearing the end of your cycle, or it could mean something clinical. The only thing to do at this point is to take a pregnancy test (at home or, if you're really worried, get a blood test through your doctor).