Could I be pregnant?

My boyfriend and I are not trying to conceive but are not trying to prevent either. we don't use any type of contraception.

I have a regular 27 day cycle and my period is always on time. My periods are always the exact same and I'm wondering because this time seems to be very different. My last period was on January 26th. I began what I believe to be my period yesterday on the 22nd so it was right on time.

However, my normal period goes like this: 2 days before my period my breasts get sore and swollen. The first 3 days of my period are a heavy flow and the 2 days following are light and slow down to spotting before they stop. I get painful menstrual cramps and back aches that only last the first day of my period.

This time, about 5 days before my period, I started getting severe headaches, which is odd for me because I NEVER get headaches, not even when I get sick. My breasts haven't been sore or swollen, except for a night or 2 before I started getting headaches my nipples were extremely tender. Whenever I would touch them or something would barely touch them I would feel a burning sensation. I woke up the next morning though and it was gone. On the 20th and 21st I began spotting a pinkish and brownish discharge that I would notice in my underwear or when I went to wipe. It was very minimal and it started to make me wonder because I've never experienced this either. However, yesterday I began what SEEMS like my period but a lot lighter than normal. I used 2 tampons yesterday and 2 tampons today. Today, I have been bleeding less and less as the day goes on. I also haven't had any back aches or menstrual cramps. I've had some mild cramps in my lower pelvis off and on, but they don't last long or feel anything like menstrual cramps. I have felt nauseous here and there, usually night time when I'm about to go to bed but it doesn't last long. I haven't felt bloated but when I look in the mirror and stand sideways my stomach looks like it sticks out a bit more than usual and feels a tad bit firm. I have been really tired and not too hungry. I've also been urinating a lot, but then again I've also been drinking more water than I usually do. Lastly, I've noticed that about this time I would be breaking out but my complexion is actually the clearest its been in a while.

I took an at home pregnancy test yesterday on the 22nd in the late afternoon and it showed a faint negative. I'm not sure if I peed on it enough, either.
I'd like to think this is my period but am doubting it considering how different it is from my normal period and I just feel different.

Could I possibly be pregnant? Should I take another test or two? If so, when should I take another test?