Need a Little Help.

I'm not sure what is going on with me, but being pregnant is one of the two things that i've gotten it down to.

On 2/26 I had sex.
On 3/8 I started my period. But it was a day early and heavier than usual.
On 4/9 I was suppose to start my period. Here I am, 9 days later and still no period.

For the past three or so weeks, i've been going to bed at like 6pm or 8pm. This is unusual for me because I am the type of person that can stay awake for days with little or no sleep. I can't be awake more than five or so hours and not feel like I need a nap now. I have little bits of cramping, like pre-period cramping but it doesn't hurt near as bad as it does before I get my period. My muscles feel strained and my sides hurt. Bending over also feels like i'm straining to do so.

The only other option I could think of that could cause me to be this late would be that I could be underweight. I'm around 5' to 5'1'', depending on where i'm being measured and I weigh between 100-105 pounds. It varies throughout the day/week. I don't think i'm eating more than usual and I don't really think i've changed my eating habits. I've never eaten more than 1,000 calories in a day, my stomach is sensitive to food.

So, any idea as to what could be wrong with me? Sorry that was so long, I wanted to make sure I described both of my "options".