Advice please! Implantation bleeding?

Hi Ladies, I am so confused...I was so certain this was my month. I have a shortish luteal phase (11 days) and I normally spot up to 4 days before my period is due. This time there was nothing! On sat (when my period was due) I tested and got a very very faint bfp! Retested on sun, still very very faint bfp (my husband wasn't convinced though).Then on Monday morning suddenly had bright red blood (tmi) so thought it was the start of my period so put in a tampon and went about my day. But it never got heavy like a normal period and today there has been nothing but slight cramps. Could this have been implantation bleeding? Too nervous to test. At what point would you go to the doctor?