since about Sept/ October I have been coming on my 'period' on the 8th ish of every month, for about a week, coming off and then around the 20th for 2 or 3 days and then off. it's was exact like that Sept/Oct/November/December and I expected it to be the same this month but I haven't came on at all! 

just after coming off (around December 22nd) me and my bf had unprotected sex and he came inside me

around the time I was due on At around the 8th I got cramping but no period; thinking it was just down to stress I didn't think anything of it but even the time around the 20th there was still nothing. 

I started getting little pains in the lower abdomen and pain in the lower left of my back which was starting to worry me, especially with the shoulder tip pains I was having. 

when I went to the loo yesterday I found little specs of blood, then a few hours later slightly more blood when I whiped, today there's nothing

I've also been getting really bad tailbone pain from time to time which is unusual because I haven't done anything  that could have damaged it

yet, despite this, the several pregnancy tests I've taken are all negative? could this be because of the implant? (nexplanon)