What Causes Spotting And Abdominal Cramps In Between My Periods?

for 4 months now ive bin spotting in between periods my last time was longer than normal my periods have bin light for months but my last period i finished on thursday by the monday i had more bleedin same again tuesday then again friday n saturday so far ive managed to last a week with nothing. also my cervix keeps tightening as well as i keep gettin the odd stabbin pain in my abdomen and same in my neck n shoulder not constant pain tho just every now and then ive done several pregnancy tests and they ve all bar one come bk negative one came bk really faint as a positive but done loads since and it said negative please help as im seriously worried and dont have clue whats wrong and its starting to come embarrassing with the bleeding