I am late but tests are negative and not sure if i have had implantation bleeding

I am not sure if im pregnant. my story is:

I came off the pill in October after taking it on and off for the last few years. i had bleeding a few days after stopping it around the 14th Oct.

Then i came on on the 16th November. i had sex on the 27th Nov, 1st Dec and 2nd Dec and 10th Dec. But to be honest im not sure how long my cycles are and what day i ovulate. Its now the 22nd Dec and i still haven't come on, but after sex yesterday i bled a little. And there was a small amount of blood in my knickers today but only small. Although i would have thought this was too late to be implantation bleeding. I took a pt yesterday and it was neg and i took one last week and that too was neg.

Any one have any ideas?

i have had sharp pains in only one of my boobs and dont think i have any pregnancy symptons. the last couple of night i have felt like i was pulling my stomache muscles or something when i was semi asleep, can't really explain it though.