Pregnant or not?

Hi,

For a few months now, my belly seems to keep swelling up more and more.
I've heard from more people about this but still don't know what it is..
When I stand up, there's nothing or almost nothing visible, but during the day my belly gets thicker and thicker.
Today, when I got up, it was already swollen and a bit hard. Now it's really very big. I got sharp stabs around my navel and have had pregnancy symptoms for quite a while now.
The last test I took was negative, the one before that too, I just got my period.
Could that belly also be caused by the pill, or am I pregnant..?