My boyfriend and I had unprotected sex and now I am about 4 days late for my period, and I think I am experiencing implantation bleeding but I am not sure. I started spotting yesterday, it started a light pink color and now it is like a burnt red. Last night it stopped completely but now it has started back with dark brown blood then back to pink and red. I'm not cramping like I would with my period but I am having lower back pain. The spotting is not like a flow and it is not enough to even fill a light pad.It just doesn't feel like my period at all.Do you think I am experiencing implantation bleeding?

Sounds like you are just having an off period. The flow should pick up in the next few days.