When will i be able to take a pregnancy test? Could i even be pregnant?

[Chrissy:)]

Ok, so yah basically...Ive been nearly 2 weeks late for my period..i don't know if i could be pregnant, maybe my periods just being irregular. but i did have unprotected sex over a month ago, a day or 2 after my period ended. i did go to the GP, and she took a blood test, which came back negative, but that was at least 1-2 weeks after i had the sex. was it to early to be detected? im really confused...my period has never been irregular. i get cramps though, but when i do get cramps, i usualy get them the day only before my period..its been about a week with light on/off cramps, bad mood swings, and i feel like im going to be sick at times...but isn't it to early to get symptoms? if i was pregnant it would be about 6 weeks into it. to early for symptoms right? please help. also will i be able to take a test? i don't want to buy a test when im short on money and it wont be able to give me a correct answer yet..