Aiyoh, this sounds stressful! Honestly, clear mucus mixed with brown blood a few days after period bisa jadi normal (like leftover from cycle), tapi if it's turning bright red and you're getting cramps plus lower back pain, better check with your doctor sebab timing tu bit unusual, could be anything from hormonal shifts to something else, and with the negative test at least you've ruled out early pregnancy, tapi doctor kena tengok properly lah. A few of my mum friends had similar scares and it turned out fine after doctor check, so jangan worry sangat first, just get it checked out soon to be safe ye?