Hello :) Looking for advice!

Hello everyone!
I'm Alix, i'm 21 and currently TTC #1 and im just looking for advice as i have had a bit of a dodgy month :S haha
This does have a bit TMI and im sorry lol, but i really need adivce so here goes.. haha!
soo.. AF was due around the 10th of this month, but i'm not always on time so it could of come anytime between a week before the 10th or a week after.
Well she didn't arrive! Then about the 20th i started having some brown spotting for about 2 days, it was very light and only when i wiped, then the next day and a half it turned into pinkish/watery spotting and a little bit heavier but nowhere near enough for a pad or a tampon, then the next day it turned back into brown spotting and was gone the day after. At first i thought it was AF coming even later than usual, but it was VERY unusual for me to have a period like this, i have a very very heavy flow and really intense period pains usually, so i did a HPT (very basic clear and simple pregnancy test strip) on the 27th and i got a BFN Then yesterday and today i have had some mild what feels like period pains but a sort of dull ache, and slight pain in my back. I have also been feeling nauseous and put off some foods!
Is it possible i could be pregnant and for some reason it isn't showing up on the test?

Thanks in advance!
xxxx

Sending baby dust to all! xx

It could be that you're pregnant and your HCG levels are just not high enough for that particular test to pick up. Maybe try one like first response? I did one of those 3-4 days before period was due and got a very faint positive. Repeated a week later and it was a very strong line. You're already late so you should get a BFP soon if you are, but if your period doesn't turn up and you still get negative results then maybe ask doctor for a blood test.
Good luck