Implantation bleeding or breakthrough bleeding?

Hi, I've been on the pill for a number of years now and have never had any problems with remembering to take it.

Basically, I had my last period (or withdrawal bleed) on the 23rd of september and it lasted a few days as usual. On the 6th and 7th of October I didn't take my pills and didn't then go on to take them afterwards, just continued taking the normal one a day. On the 7th I had unprotected sex with my fiance.

A few days later I had nausea, cramping and dizzyness. I also had fatigue but that happens a lot due to depression.

Then last night on the 14th I had a very small bleed, literally just after wiping on the toilet once, that was it.

I'm not sure if it is breakthrough bleeding though because i normally get my withdrawal bleed 4 days after my last pill (of 21) when i go on my 7 day break. but the bleed i had last night was 8 days after my second missed pill.

This, along with the symptoms (which also now include sore breasts), makes me think that it was implantation not breakthrough.

I'm not due my period until the 21st so  not for 5 days but i seriously feel like im pregnant.

I took a test today but it was negative, i'm guessing that's just because its too early though.

can anyone shed any light on my situation?