What Causes Delay In Periods?

hi,

i am 7 days late on my period, i have done a test 4 days ago witch came back negative.... but the thing is i feel sick, i need the loo more often,my boobs are sore, my tummy is swolen an i am very tierd alot... also every now and again i am getting a pain down my left side.... what do u think i should do about this??