BFN, spotting, backache, CD56

i'm only 18, not trying to conceive but i just wanted a bit of advice from people who have experience so anything is helpful

i had unprotected sex once and i've been worrying like hell ever since, at first i thought my period was just late because i've been stressing about it every day since. i've done 3 pregnancy tests all negative, first 2 were when i was 1 week late (my cycle is usually 30days exact) and my third was yesterday on CD56w when i was 22 days late

i've had back ache on the left side for a few weeks but nothing too painful that prevents me from going about my day. 

i started getting spotting on sunday night (2 days ago) and the colour has varied from bright red to brown

any help or opinions???