Help! Pregnant or not?

So it's been about 8 days since sex without protection, I have the most tender boobs ever. I've been really tired and been really sick, but haven't thrown up. I've been having weird cramps and my lower stomach is a little hard. I took a test but it came up negative. When should I take another one or am I not pregnant? It's about a week before my next period I think. (My periods are irregular).