My period is a week late. Can I possibly be pregnant?

I began having intercourse with my boyfriend on Friday July 18th, and it sort of became addicting and so it continued for the following three days. We've both been tested for STDs so I am in no way worried about this, however, as stupid as it is I did not use protection. As of late, that is, this week, I've been feeling really weird feelings in my lower stomach where menstrual cramps usually are. On Saturday I had feelings similar to menstrual cramps but they would subside and come back. My period is a week late, I woke up this morning with an unexplained headache, I've been peeing like crazy, I've had increased discharge, my nipples are sore, and to add to all of it, the other day I ate oysters, lucky charms, and chips and salsa and am eating more than I've ever eaten in my life. On Tuesday I used a pregnancy test and it gave me a negative sign, BUT, that was the week that I was expecting my period, so I don't know how reliable that is. Luckily the pack came with 2 extras.

My stomach has never felt weird like this before, and that was not the first time I had sex, but it was the first time I did without a condom. I have never had such insane mood swings, and I've been stressing out, too. As for the headache I am not going to take any headache medication because I don't want to put anything in my body just in case I AM pregnant.

What are my chances? If my probability is high, should I continue secretly using the home pregnancy tests or am I actually going to ask my mom to take me to Planned Parenthood? FYI she knows I am sexually active, however she has no idea that I am still having sex.

Help me out here! :O :confused: